On my 2.8GHz Athlon 64, 20 eight-limbed figures brings the framerate down to 400 from 2200 with one. I know that's not exactly a problem by itself, but once you throw in the rest of the game code, including other CPU intensive things like AI, the framerate could eventually fall below the refresh rate.
Then you have to consider different CPUs. There are still plenty of people with 2GHz P4s, which are approximately half as powerful as what I have(I'm talking about real world performance, not clockspeed).
Maybe I'm just being paranoid about this, but I still think it could effect older computers tremendously. Besides, a deactivate feature should probably be added just for consistency with the rest of the behaviors. File I used to test speed - .99.4