Wanna know the primary reason why Pixar, ILM, Imageworks, Weta, etc use linux renderfarms? It's free and it was the most logical platform jump for all of the shops that started out with SGIs running irix.
Not to mention all the other apps we rely on are either commercial or proprietary and supported by a significantly large team of engineers. Any open source stuff anyone uses is at the OS or utility level.
We use gimp for quick and stupid stuff (like maya shelf icons), or launch Photoshop w/ emulation.
Anyone doing serious paint work uses Photoshop on OSX. Anyone doing serious editing work uses Avid on windows or Final Cut Pro on OSX.
It's not that linux is useless, it isnt.
But alot of those open source 3d, editing, paint, audio apps are well, pretty much useless to the 'Pro'.