As an uncontroversial example, maybe someone is well qualified to manipulate explosives in their garage. But society would still say the risk is not worth it, because by the time the “nuisance” is dealt with, there would be unacceptable losses.

The relevant measures to be discussed are what is the probability of someone using PCP and not being a nuisance, and if they did use PCP, what severity would the nuisance be?

A little thing called democracy. Perhaps you've heard of it.

The problem is that taking pcp significantly raises the probability the user will commit an act of violence, by which point it’s too late.