I don't think a lot of you understand the premise. It doesn't make any money, or lose anyone any money. Even if the service will exist, it'll simply serve as an initiative for people to buy more games. A failsafe, if you will. Say I bought... Torchlight. I play it, and I dislike it. Currently, that's it. I have torchlight registered to my steam account permanently.
With this new system, they can give people an incentive to buy new games because hey, at least they can use that credit to get a new game that doesn't suck, right?
To me, it sounds like a good idea. Even though a lot of you dolts are talking about "well what if someone trades in a game they got for free!" you don't seem to factor in that it's simple to keep a server side representation of every game you've bought and how much for.
As for cracking games, yeah, that's a risk. But if someone was going to pirate a game like that, they could just as easily torrent it. Trying to fight piracy is redundant. Steam solves a lot of multiplayer issues, what with Steam verification being needed to start a steam purchased game, but on the whole, fighting piracy is fighting fire with cigarettes. There's only so much you can do.