While that's true, it'll lead to the creation of more games for Linux, particularly with a lot of people apprehensive about the big GUI changes in Windows 8.
More importantly, 90% of my "regular" gaming takes place in Source-based engines (TF2/L4D2), so I would be more than happy as long as those Source games are available on Linux (and I believe that was Valve's plan all along with Steam for Linux).
And Steam has been pretty good at picking up pre-existing Mac ports, notably Civilization IV, Assassin's Creed 2, and several Star Wars games. I think Steam is also largely responsible for the Mac port of Civilization V being a first-class platform instead of the port being outsourced and late (though cross-platform multiplayer support took a while, and so did some of the DLC).
That said Steam on Linux cannot be a bad thing, I think it helped bringing quite a lot of games to OSX as stated elsewhere in this thread. The bad news is that most of the times it means Cider (http://transgaming.com/cider), so the result pale in comparison to rebooting on Windows. I expect the same to happen on Linux: many ports using Wine...