Just want to draw some attention to the steambox that allegedly will be running Linux that is probably also a great part of the motivation for porting everything to Linux. That being said there are likely also some truth in the argument that Valve are preparing for the future, since Microsoft are slowly trying to get people on the windows-store and lock stuff down this seem like a good way to improve your options for Valve, and gamers in general.
As far as I remember an employee from Valve announced that the Linux client is more profitable than the Mac client.
And for all of us using Linux already, all of this is just great news.
I definitely agree with you. I think Valve's primary motivation for porting Steam (and an ever-increasing number of games in their back catalog) to Linux is the Steam Box, or whatever it is they call their console. There is probably some measure of future-proofing involved too, but Valve is a business and I don't think they would invest the resources into porting everything to an operating system with such low desktop marketshare for future-proofing alone. In any case, it's a win for Linux users. Valve has already ported most of my favorite Source games. I will be estatic when Portal 2 is released for Linux!