It doesn't make any part of the body weaker, and all parts of the body have the same level of numbness, depending on your body, how much you take, how quick, blah blah blah...Yes but it does seem to make the lower extremities weaker. This is where they have been told to attack. Mind you if you want to disable someone, dislocation of a knee will do it.
It's possible that actually disabling someone through the legs may be more effective, but I don't know of any actual research on that. Regarding dislocating a knee, on most people that will work. On someone who is high on PCP, that's not necessarily the case. The lack of pain means you can dislocate their knee, break their bone, whatever, and they (may) just keep coming at you. Afterwards, though, they'll regret it. Telling someone that attacking there knee will make them disabled is very dangerous, my advice (unless you're a LEO) would be: "You know someone is on PCP and attacking you? Run."
Incidentally, from what I have heard the same thing can happen with crack/cocaine, depending on the individual in question and how they react to it.