I dont understand this, Valve's Source engine is not even ported to OpenGL renderer for Mac, what they used instead used a OpenGL wrapper to the engine pretty much like WINE and Cedega (I hope it is not WINE). The engine still itself interects with DirectX.
That's why they did not provide OpenGL renderer for Source engine games on Windows like Blizzard Entertainment's game, which is pretty much same thing, will cause additional overhead instead of increasing performance.
If their engine is not native to the specific platform than We can use WINE instead.
WINE doesn't support multithreading version of Source Engine, so it's important to have native one (even if it will use some wrapper).