A beefy workstation can be handy during development -- e.g. if an artist is working on a million polygon mesh in 3DS Max, then they'll want a good GPU and lots of RAM, etc... I personally recommend getting an SSD in your development PC, because it makes loading large amounts of data (or many different applications) very fast.
However, it's also handy to have a crappy old PC around the place (even if it's not your main one) for testing your game. For compatability testing, it's also good to have PCs with Intel, nVidia and ATI graphics cards, etc, etc...
Sometimes if your main PC is too powerful, then you won't realise that your game is a resource hog, and when you run it on an "average" PC, you'll be getting 15 frames-per-second rather than 60
For really intensive jobs, like rendering/baking out data from 3D programs that takes hours, you'd usually use a "farm" of PCs, rather than a single PC with 100 CPUs. At my last job, all the artists would leave their PC's on overnight while running a program that connected them to a "render farm" network. A main server would go through a queue of jobs that needed to be rendered, and would pass out small chunks of work to all the different computers that made up the farm, and then merge their results together. This gave us the equivalent of a giant super-computer without having to actually buy one, plus we could add more power just by connecting more regular PC's to the network.