Even if you eliminate the typing difficulties (and they are quite severe) of the platform, you still haven't replaced the mouse. On the PS3 it's tedious maneuvering of a pointer with a joystick (second order control system == frustration), on the Wii it's an arm-killing posture that is also aggravatingly imprecise.

But taking all of that out of the equation - assuming that magically the Wii and the PS3 are both capable of interacting with USB mice and keyboards... how many people do you know who have an (extension cord required) USB connection to a keyboard and mouse combo on their couch? I can count these people on one hand. In fact I can count them on 3 fingers, and all are pretty hardcore tech geeks to boot.