While I seriously doubt that PS/2 uses more system resources than USB, there are some definite reasons why I choose PS/2:
1) Higher refresh rates. I went from PS/2 to USB and now back to PS/2. I'd forgotten how wonderful a 200 hz mouse rate was. 125 just doesn't cut it.
2) No "freezing." For those of you on USB mice... Does your mouse freeze occasionally when a program first starts? PS/2 never does that to me, while USB did. Same system, same OS install, only thing changed is the mouse.
3) Compatibility. Ever had to boot into safe mode while using a USB mouse? Bet you had fun using keyboard shortcuts again. How about alternative OS's? Ever tried to install Linux with a USB mouse?
USB is great for other things (scanners, printers, etc) but for keyboard & mouse I'll stick to PS/2, thank you.
Viper GTS