If Nvidia have the only working drivers for Source on Linux, then they can push up prices on their hardware to Valve, safe in the knowledge that it could take months for Valve to port their entire games library to AMD or Intel. If Valve keep support for all three, then they can squeeze the hardware manufacturers for better prices lest they take their custom elsewhere.
I don't know. I think "Drop your price, or we'll stick an Intel GPU into our new console" is a pretty bad bluff, even if they can get Left4Dead running on it surprisingly well. NVidia and AMD should both know nobody would ever actually use an Intel GPU for something like that.
It's a little more nuanced than that. It can put the brakes on Nvidia and AMD from trying to screw the nut in pricing, especially if the entire Steam catalogue will play on a good enough Intel GPU.
If Nvidia have the only working drivers for Source on Linux, then they can push up prices on their hardware to Valve, safe in the knowledge that it could take months for Valve to port their entire games library to AMD or Intel. If Valve keep support for all three, then they can squeeze the hardware manufacturers for better prices lest they take their custom elsewhere.