Dunno, I can't say I've ever heard someone say "If only Mac had this feature, I'ld switch over". Even with steam coming to Mac, did anybody change OS? This is a different situation with Linux, there are people waiting to make the switch.
Silly comparison IMO.
Perhaps you don't hear it that often is due to Mac OS being another even more expensive closed source OS if you already have a Windows PC, that will not only drive your equipment cost up, but also the update cost.
If I were Valve, I'd probably have a run-of-the-mill gaming hardware setup with Steam on top of linux. The linux games would be labeled with a "Works on Steambox" seal of approval. All other games would require a Windows computer that's capable of running the game on the same network. The PC would then run the game sandboxed and stream everything to the console in an OnLive style gaming setup without the OnLive style input lag.
That statement doesnt even sound right, with opengl 2+ you compile your shaders so that you can use to them to draw. Compiling is a one time thing, your not doing it during draw calls. I say its bogus.
That's at the API level. Different hardware does different things at the driver level. On most NVIDIA hardware, for example, shader constants are injected into the shader binary at draw call time by the driver (known as shader patching). This can be a substantial performance hit - substantial enough, that many PS3 games manually do the constant patching on SPUs instead of leaving it to the firmware.
It's not as easy as compile once, use anywhere. The compilation done at the API level is more of an intermediate step, with additional work done at runtime.