By helping out the FOSS community to have better OpenGL driver support it will get Valve Software a lot of goodwill. Projects like Dolphin will benefit from it, and of course those Steam Linux games will as well.
I really want to see the Linux Gaming Platform take off, many game developers avoid Linux ports because of buggy OpenGL drivers and bad support of it. I think Microsoft did this with DirectX standards because I remember early on DirectX drivers were buggy and game companies wanted to stay with DOS based games and not port to Windows, because DOS had better video support drivers. Once Microsoft worked with video card OEMs to improve their drivers and DirectX support, the video game companies ported their games to Windows 32 bit format (Windows 9X and then later NT/2000/XP) because of DirectX based libraries and better drivers and fewer bugs. It will take a company like Valve to coordinate the OpenGL stability between OEMs to get it stable enough to port games to Linux.