Hmmmm...Certainly sets how the Fey see us in one way as a trade off.
But demon's of hell are known for having rigged deals just as the Fey's themselves if I remember correctly. So I wonder what actually was the deal set up and why haven't these two great powers tried to fight each other yet?
Then again, who's to say how well the stories really relate to reality? It could be significantly more complex then a simple story could explain that Aggie was telling Hunter. Still the Prince is right, that isn't fair having someone else pay your debt for you.
I could mention one of the interesting points in The Dresden Files series; they have demons and faeries and wizards and vampires and all sorts. The line goes something like, if you make a promise, your power is tied into that. If you _break_ that promise, you lose some of your power, and the person you promised gets control over you.
Depending on how powerful the person you gave your word to is, and how evil, crazy, or just plain cruel they are, this may be a very very bad thing.
I can provide the examples given in the books, but, just to put you in the right field... if you had given your word to a being with almost godlike power and a head that was possibly not screwed on right... wouldn't you want to redeem it and get the heck away from them?
Demons suck. Imma kill them. Of course, the fae are losers for ever dealing with them in the first place.
Ha! Take that fae! I, a human, am SMARTER THAN YOOUUUUUUUU!! ;P