Once enough games are built for Linux, why would anyone use Microsoft Windows for gaming?
Because Windows is there from the moment a new PC is first turned on. For the hard-core enthusiast crowd, that purchases components and builds their PCs by hand, it may increase the likelihood that they will see Linux as a viable option for their build, at least as a secondary OS. For Joe Normal, Windows *is* the computer. Average consumers do not distinguish between OS and hardware. To them, a Windows PC is a PC, unless they are educated as to the existence of alternatives, which is still rather difficult, mind you.
Not that I don't support what Valve is doing. If that can have some success, we could potentially see a future where Linux-based game consoles start popping up, with Steam installed OOTB, and a variety of other FOSS games available for download.