The FPU is rare because it was optional. My 030/50 was factory sealed and had the FPU seperate and had to be installed. I think the speed of the chip is uncommon also. Its just like finding a 40mhz FPU for a 386/40 AMD system (Yes I have an FPU on mine) compared to the more common 386/33 FPU chip.
The FPU does spead up zipping and unzipping large archives on the amiga, not sure if games use it for anything.