It might be more likely that this will be another option for publishers to promote their games, similar to a free weekend. Make it available for sharing for a short period, so people try it for free and Valve gets to mix it into their social network shit, by giving out a badge for sharing X games with friends. I guarantee that will lead to completionists buying sharable games SOLELY to earn the badge(s), so Valve would almost be double-dipping then.
But yeah, I doubt you'd be able to freely choose which games you want to share with no restrictions except that no two people can be playing it at the same time... Like others on here, I have hundreds of games I don't currently have installed, so that's a lot of potential free-loading in lieu of actually buying games. They'd have to at least limit it to one slot for a shared game at a time, and maybe give you a few more as a reward for leveling up.
This comment was edited on Jun 19, 2013, 12:48.