In general I would say yes (assuming the work can be distributed evenly). Multiple systems means you have duplication of more system components such as L1/L2 cache, FSB, drive controllers. However, you pay for the extra hardware.
This brings up another question. Are there techniques for distributing tasks evenly over a network of computers? I'm aware of rendering farms and beowulf clusters (though I don't actually know much about them), but is there some way to implement a similar idea with PD? For instance, I could set up two comps for 3D and one for audio--but if the audio computer isn't being used to its fullest capacity, is there some way for it to take a load off the others? I realize that with 3D this would be impossible because of the speed of a network; what kind of multimedia tasks could take advantage of something like this (if any)?
-Ian