I polled some judges on Twitter, and unless a lot of us are mistaken, this card is busted with Jeskai Ascendency. It's not as busted as it could be because of how the card is templated.
The ability that lets you draw cards is tied to giving it to your opponent. Therefore as soon as you draw cards, you have to give defector away. However, if you have an Ascendency and an instant in hand, you can tap Humble Defector and respond to his ability on the stack by playing an instant and untapping him. This allows you to put another instance of "Draw two cards. Target opponent gains control of Humble Defector" on the stack. As long as you can keep playing instants and putting instances of this ability on the stack, you can retain priority. As soon as you stop doing this, you draw 2x cards and give control of Humble Defector to target opponent.
See what I mean about "not as busted as it could be"? You don't have access to any of the cards you draw with his ability while you still control him. If your hand is empty, you draw two cards and give him away. However, if you have even 1 instant, you can draw four, accessing those cards at the same time you relinquish control of Humble Defector. If you want to keep them from benefiting, maybe Jeskai Ascendency decks could start running Retraction Helix and Daring Thief. Wouldn't that be a hoot?
If you could chain the instants you draw with his ability into more untaps, that would be insane. This card isn't insane, just quite good. I don't know how good he is outside of this scenario - cards that let your opponents benefit as well are generally not as good. Still, drawing cards in red is typically done by "looting" and this could find a card that kills them before they can even use him.
Could this be worth a buck or two? I think the hype around him doesn't preclude that. I think he may be better in the Modern version of the deck which has more cheap cantrips, but with Fatesticher and Faerie Conclave running around, I'm not sure there's room, at least not until someone retools the deck. All I know is this is one of the only uncommons I see so far with the potential to be worth money, and I bet the foils are dirt cheap, initially. Watch this card closely.