I think having a lot of new competent programmers (like gaming company programmers) on your OS results in bug-fixes, performance improvements, and "itch-scratching" that will improve the Linux desktop experience.
I can't think of the downside to gamers, but I can think of the upside to Linux.
Because game companies will likely delay games, cut corners to get cross-platform and claim its too hard, but still charge the same price for less content.
Most indy studios use a tool like Unity or gameplay3d to build games, which can already target Linux.
Triple AAA titles from studios like EA already have OpenGL ports for the PS3/PS4, so it's just a matter of adding a layer to interact with X and the input APIs. With their huge amount of resources and budgets, this is child's play.
I can't think of the downside to gamers, but I can think of the upside to Linux.