Yeah, I don't see gamers switching to Linux specifically for the games — just like they're not switching to Macs specifically for games, and Steam + Valve games have been ported there two years ago. It's about tapping the market of people who don't want to switch to Windows specifically for the games. And just maybe a preparation for a rumored Valve/Steam console.
I have Windows installed just for games, but not having to reboot for playing would be awesome. Although I'll have to wait and see… I really, really don't want to install those PITA binary blob graphics drivers anymore and if the open source drivers won't perform well enough, I may need to keep using Windows for those games that are available. Now, Windows is a giant PITA too, but at least it's compartmented away…
And as for the selection of games, having a well established game sale/distribution platform coming to Linux might be an incentive for more publishers to port some of their games.
The final burning question is: Will Team Fortress 2 be ported? And will Linux gamers get Linux themed hats? Because that's what it's really all about.