Originally Posted by Scorpion667
There's your answer.
Many usb mice are not compatible with PS2 adapters. Not even the Microsoft intellimouse series which is quite old.
Some USB keyboards however will benefit from using over PS2, such as my 6gv2 which has no ghosting when hooked up with PS/2 adapter, meaning that if I press every key, they all register. On usb I think it has a limit of 6 simultaneous keys. Not that it really matters =P
Plus, PS/2 doesn't have to be reported every x miliseconds (doesn't have a polling rate like the USB port), it's sends the signal right away on actuation.
The lag for keyboards is less on PS/2 because a key press doesn't have to wait to be reported in the next poll and therefore uses less resources as the cpu only works on key actuation, however the datapath for mice is inconvenient due to the higher amount of information it reports when moving and because mice have more advanced features that require the wider USB datapath like specific hardware recognition, saving macros, profiles, changing cpi, etc.
That's why some brands insist on using usb on keyboard so they can add those profile gimmicks and stuff you probably won't need, despite the fact that PS/2 is superior for that kind of hardware.