Among other things, porting their software to Linux might bring other fruits:

* You can run many instances of a dedicated game server (roughly in the hundreths mb of ram usage) on the same OS, or through virtual machine so it could be moved.

* Gaikai/OnLive live kind of game streaming - again no cost for OS - so they can run each instance of the game per one installed OS and stream video for the rest.

* Console - probably customized version of Linux, and if possible the game running in ring 0 with the kernel. Drawing almost directly to the hardware, without going to API's, user and then kernel functions. (Avoid vertex/index buffer checks, and many other things).