This would only be advantageous to a Maya or Cinema 4d setup where animation is involved. Each machine out of the 36 can each independently set up the entire scene, and work on a seperate frame, and then send it back to the master machine which compiles them together. So even if it takes a Mac Pro 2 seconds to do 1 frame, and one of these 20 seconds, it is still faster because in 20 seconds, you actually get 36 frames out of it, not just 10. As long as a Mac Pro is less than 18 times faster per core, these are
technically faster.
However, under load a Mac Pro consumes about
200W-250W , vs
about at least 50W per G4, which adds up to at least about 1800 Watts, but most likely not more than 3600W. With about 10 times the power consumption, you're paying a significant amount just to run a full farm of these, and I doubt that they would be significantly faster than Mac Pro's enough that getting a few extras wouldn't be enough to settle any potential gap.