I buy every game that I spend a reasonable amount of time with. I just might not buy it on the platform for which I originally played it.
For example, I've bought Skyrim on Steam already, but it's totally gonna leak for 360 a week early, and that's probably where I'll play my first playthrough.
If I was a publisher releasing multiplatform games on Steam, I would ask Valve to go ahead and release the game on Steam once it leaked for other platforms. I see no good reason why paying customers should have to wait longer to play a game because they want to stay legitimate.