I do not understand. What is the appropriate use case for a Mac Pro where something equivalent and much cheaper cannot be used?
I'd switch to Linux if the apps were there, but it's not even close. And I'm an edge case, I suspect most creative types would rather not mess with xorg.conf.
I have ran some windows audio applications on Ubuntu 10.04 and the timing accuracy (tested by specialist software) was several orders of magnitude higher. In addition, I can achieve higher workloads. This is all with software that wasn't even compiled for my OS.
Anything that needs workstation-level graphics like the NVIDIA Quadro (for doing stereoscopic graphics, multi device synchronization, broadcast pipelines, and more).