The bigger purpose/politics behind the Kavacha Kundala of Vasusena Karna.

The armor and earrings of Karna being taken away is a very famous incident in the Mahabharata of Vyasa, at the very least for the character of Karna it is considered one of his highlight moments.

Also first of all Duryodhana admitted that it was not Indra who was begging to karna for this kavach but instead it was Karna who was begging to Indra for the Vasavi dart.
Quote:

The dart that had been begged (of Shakra as a boon) by Karna for the slaughter of Arjuna was baffled by thee through Ghatotkacha!

Analysis: Karna probably begged like a needy person because he knew he can never defeat Arjuna in a fair battle. What a coward.
Source: http://www.sacred-texts.com/hin/m09/m09061.htm

Cross references of his Kavacha Kundala trade (all outside of Vana Parva).

1st reference: http://www.sacred-texts.com/hin/m01/m01112.htm

And Indra desirous of benefiting his own son Phalguni (Arjuna), assuming the form of a Brahmana, approached Vasusena on one occasion and begged of him his natural armour. Thus asked Karna took off his natural armour, and joining his hands in reverence gave it unto p. 237 [paragraph continues] Indra in the guise of a Brahmana. And the chief of the celestials accepted the gift and was exceedingly gratified with Karna's liberality. He therefore, gave unto him a fine dart, saying, 'That one (and one only) among the celestials, the Asuras, men, the Gandharvas, the Nagas, and the Rakshasas, whom thou desirest to conquer, shall be certainly slain with this dart.'

2nd reference - http://www.sacred-texts.com/hin/m03/m03308.htm

"Thereupon, O king, taking the blazing dart, Karna began to peel off his natural mail. And beholding Karna cutting his own body, the entire host of celestials and men and Danavasset up a leonine roar. And Karna betrayed no contortions of face while peeling his mail. And beholding that hero among men thus cutting his body with an weapon, smiling ever and anon, celestial kettle-drums began to be played upon and celestial flowers began to be showered on him. And Karna cutting off the excellent mail from his person, gave it to Vasava, still dripping. And cutting off his ear-rings also from off his ears, he made them over to Indra. And it is for this fact that he came to be called Karna. And Sakra, having thus beguiled Karna that made him famous in the world, thought with a smile that the business of the sons of Pandu had already been completed. And having done all p. 600 this, he ascended to heaven. And hearing that Karna had been beguiled, all the sons of Dhritarashtra became distressed and shorn of pride. And the sons of Pritha, on the other hand, learning that such plight had befallen the son of the charioteer, were filled with joy."

3rd reference - http://www.sacred-texts.com/hin/m05/m05169.htm

Without sense, this one hath been deprived of his natural coat of mail. Always kind, he hath also been deprived of his celestial ear-rings.

4th reference - http://www.sacred-texts.com/hin/m07/m07176.htm

Indeed, that dart, O king, which he had kept and adored for years for (achieving) the slaughter of Pandu's son in battle, that foremost of darts which Sakra himself had given to the Suta's son in exchange for the latter's ear-rings, that blazing and terrible missile twined with strings and which seemed to thirst for blood, that fierce weapon which looked like the very tongue of the Destroyer or the sister of Death himself, that terrible and effulgent dart, Naikartana, was now hurled at the Rakshasa.

5th reference - http://www.sacred-texts.com/hin/m07/m07177.htm

For thy good, Karna was divested of his ear-rings by Sakra with the help of an illusion. Similarly was that subjugator of hostile towns deprived of his (natural) armour. Indeed, because Karna, cutting off his (natural) armour and his brilliant car-rings, gave them unto Sakra, it is for that he came to be called Vaikartana. Karna now seems to me to be like an angry snake of virulent poison stupefied by power of incantation, or like a fire of mild flames. From that time, O mighty-armed one, when the high-souled Sakra gave that dart unto Karna in exchange for the latter's ear-rings, and celestial armour, that dart, viz., which has slain Ghatotkacha, from that time, Vrisha, having obtained it, had always regarded thee as slain in battle!

6th reference - http://www.sacred-texts.com/hin/m12/m12a005.htm

When, for thy good, the Lord of the celestials begged of him his (natural) coat of mail and ear-rings, stupefied by celestial illusion, he gave away those precious possessions. Deprived of his car-rings and divested of his natural armour, he was slain by Arjuna in Vasudeva's presence.

Karna followed all of Surya's orders and acted exactly the way Surya told him to.

Surya's advice part 1: http://www.sacred-texts.com/hin/m03/m03299.htm

Therefore, when the lord of the celestials will ask the ear-rings, thou shouldst, O son, refuse him! Repeating again and again answers fraught with various reasons, thou wilt, O sinless one, be able to remove the eagerness of the lord of the celestial for the possession of the ear-rings. Do thou, O Karna, after Purandara's purpose by urging answers fraught with reason and grave import and adorned with sweetness and suavity.

Karna's act part 1: http://www.sacred-texts.com/hin/m03/m03308.htm

Hearing these words, Kama, said, 'O Brahmana, I will give thee homestead land, and fair damsels, and kine, and fields; but my mail and ear-rings I am unable to give thee!'" Vaisampayana continued, "Although thus urged with various words by Karna, still, O chief of the Bharata race, that Brahmana did not ask for any other boon. And although Karna sought to pacify him to the best of his power, and worshipped him duly, yet that best of Brahmanas did not ask for any other boon.

Surya's advice part 2: http://www.sacred-texts.com/hin/m03/m03300.htm

'If O son, thou givest away thy ear-rings to the wielder of the thunder-bolt, O thou of mighty strength, thou shouldst also, for the purpose of securing victory, speak unto him, saying,--O thou of a hundred sacrifices, I shall give thee ear-rings under a condition.--Furnished with the ear-rings, thou art certainly incapable of being slain by any being. Therefore, it is, O son, that desirous of beholding thee slain in battle by Arjuna, the destroyer of the Danavas desireth to deprive thee of thy ear-rings. Repeatedly adoring with truthful words that lord of the celestials, viz., Purandara armed with weapons incapable of being frustrated, do thou also beseech him, saying, 'Give me an infallible dart capable of slaying all foes, and I will, O thousand-eyed deity, give the ear-rings with the excellent coat of mail!' On this condition shouldst thou give the ear-rings unto Sakra. With that dart, O Karna, thou wilt slay foes in battle: for, O mighty-armed one, that dart of the chief of the celestials doth not return to the hand that hurleth it, without slaying enemies by hundreds and by thousands!'"

Karna's act part 2: http://www.sacred-texts.com/hin/m03/m03308.htm

Hearing these words of Indra, Karna was filled with delight and seeing that his purpose was about to be accomplished he approached Vasava, and intent upon obtaining a dart incapable of being baffled, he addressed Indra, saying, 'Do thou, O Vasava, in exchange for my coat of mail and ear-rings, give me a dart incapable of being baffled, and competent to destroy hosts of enemies when arrayed in order of battle!' Thereupon, O ruler of earth, fixing his mind for a moment on the dart (for bringing it there), Vasava thus spake unto Karna, 'Do thou give me thy ear-rings, and the coat of mail born with thy body, and in return take this dart on these terms!

Proof that Surya was pro Arjuna

Incident 1: [http://www.sacred-texts.com/hin/m01/m01002.htm]

When Bayu together with Indra and Suryya united as allies for the success of the sons of Kunti, and the beasts of prey (by their inauspicious presence) were putting us in fear, then, O Sanjaya, I had no hope of success.

People might say that Dhritarashtra was talking about the sons of Kunti so he included Karna, but the truth is that he also said that he had no hope of success. So he was talking about something negative towards the Kouravas (the side that Karna was supporting).

Proof that Indra tried to kill Arjuna

In Khandava-Daha:

Then Sakra the lord of the celestials, riding on his white elephant, rushed at those heroes, and taking up his thunderbolt which could never go in vain, hurled it with great force. And the slayer of Asuras said unto the gods, 'These two are slain.' Beholding the fierce thunderbolt about to be hurled p. 444 by their chief, the celestials all took up their respective weapons.

Logic suggests that Indra was intending to kill them (Krishna and Arjuna) as he says they will be killed. http://www.sacred-texts.com/hin/m01/m01230.htm

Indra already knew that Surya had met Karna before he would approach him to take his armor:

Thereupon Sakra replied, 'Even before I had come to thee, Surya had known of my purpose and without doubt, p. 599 it is he that hath unfolded everything unto thee! O Karna, be it as thou wishest! O son, except the thunder-bolt alone, tell me what it is that thou desirest to have!

So Indra was already prepared to give something to Karna. http://www.sacred-texts.com/hin/m03/m03308.htm So it was probably a plan by Surya to make Indra give a celestial dart to Karna. As Karna was asking for what Surya wanted Karna to have not what Karna himself wanted to have.

Even Karna had the sign of the Vajra on his palms, indicating that he was a bhakt of Indra.
Quote:

He became exceedingly cheerless and unable to stay before Karna. The son of Radha then, pursuing Yudhishthira, the son of Pandu, cleansed himself by touching him in the shoulder with his own fair hand (the palm of which was) graced with the auspicious signs of the thunderbolt, the umbrella, the hook, the fish, the tortoise, and the conchshell, and desired to seize him by force. He then remembered the words of Kunti.

Analysis: 
It is clearly written that Karna's arms were adorned with the sacred symbol of the thunderbolt, also Indra is considered the ideal king on earth. So Karna being a king would indeed make Indra an inspiration for himself.
Source: http://www.sacred-texts.com/hin/m08/m08049.htm

Even Karna indirectly tells his father Surya that it was not Indra who cheated him rather it was Indra who was cheated by me.
Quote:

And by dispelling on the field of battle the fears of the affrighted that may beg for their lives, and relieving old men and boys and Brahmanas from terror and anxiety, I will win excellent fame and the highest heaven. My fame is to be protected with the sacrifice of even my life. Even this, know thou, is my vow! By giving away such a valuable gift to Maghavan disguised as a Brahmana, I p. 587 will, O god, acquire in this world the most exalted state.'"

Analysis: Even karna had a lot of things to gain out of this kundala sharing incident.
http://www.sacred-texts.com/hin/m03/m03298.htm
It also helped in the loss of fear that Yudhishthira was plagued with:

"What, O Brahmana, was that great fear entertained by Yudhishthira in respect of Karna, for which Lomasa had conveyed to the son of Pandu a message of deep import from Indra in these words, That intense fear of thine which thou dost never express to any one, I will remove after Dhananjaya goeth from hence? And, O best of ascetics, why was it that the virtuous Yudhishthira never expressed it to any one?"

As we can see through this incident many things happened.
But it seems like the only people who fail to look deeply into this incident to find out what was accomplished truly, are just karna fans. They have not understood Mahabharata and have not learned anything from it.
These are things that occurred due to the loss of the Kavacha Kundala.

1. Karna won great fame.
2. Karna gained the celestial sakti dart of Indra.
3. Indra removed the fears of Yudhishthira.
4. Ghatotkaca was killed so Krishna did not have to kill him.
5. Karna succeeded in getting at-least one major kill to his name, killing a warrior like Ghatotkaca.
6. Due to Ghatotkaca's death another aukshouni of troops in the kuru army perished with him.

So it was all a win win situation for everyone, both sides (Kaurava/Pandava/Arjuna/Karna) benefited from this. So don't try to use it as a reason to overrate Karna and underrate Arjuna.

Krishna to Arjuna:

Hidimva's son also, I have slain by the employment of means, viz., through Karna with his dart. If Karna had not slain him with his dart in great battle, I myself would have had to slay Bhima's son Ghatotkacha. From desire of benefiting you, I did not slay him before. That Rakshasa was inimical to Brahmanas and sacrifices. Because he was a destroyer of sacrifices and of a sinful soul, therefore hath he been thus slain. O sinless one, by that act as a means, the dart given by Sakra, hath also been rendered futile.

http://www.sacred-texts.com/hin/m07/m07178.htm
It was clearly not taken away to help Arjuna. Instead it was taken away to help Karna the most, then Krishna and Yudhishthira. Do you honestly Indra would not give Arjuna a weapon to counter this? Like Sammohana was impossible for Karna to counter?

But just like Hastinapore was the kingdom of blind people (as their king Dhritarashtra was a blind man) so are Karna supporters are blind to the obvious facts brought to the table when it comes to the Kundala-harana incident.
Proof that Indra also saved Karna's life.
Kunti to baby Karna:

And may the Adityas and the Vasus, the Rudras and the Sadhyas, the Viswadevas and the Maruts, and the cardinal points with the great Indra and the regents presiding over them, and, indeed, all the celestials, protect thee in every place!

Analysis: The words are clearly stated their "protect thee in every place". Indra never wronged karna, karna must have been indebted to Indra.
Source: http://www.sacred-texts.com/hin/m03/m03306.htm

By the way it was only his ear-rings (kundalas) that mattered not his kavacha (armor). Another thing you can learn from this is that Indra did not even care about Arjuna,a s he desired his death, he had another purpose for going to Karna. And that was to kill Ghatotkaca and save Krishna.