For, me it's the resale factor. I had my fun with Half-life 2 and CS:Source months ago, but I've never wanted to try and sell it because I'd have to give up a steam login/email address. It's not a big deal this time because I snatched up a copy off of here for like $18 when someone was selling them in mass quantity.
I'm more inclined to the usual methods of online play and verification. If I want to sell my copy of UT2K4 all I need is the cd key for online use and that's that. Same for about any other game. But with Half-life 2 I need to give up more info and make more sacrifices. On the other hand, the pirates who cracked the game can let any retard download it and play with ease even though we jump through hoops to be legit. I honestly think it's a step in the wrong direction for the time being. I'd have no problem with Steam if they made it easier to sell a game(like, not losing your login!), but why would they do that? I already bought it why would they help me sell it?