Among extremists, I think we can distinguish two types: the witness and the scold.
The witness is a living testament to the truth of what he lives. The one truth to which he clings serves as both his own path to the fullness of truth and a sign to the rest of society that we not forget that truth.
The scold insists that everyone cling to his one truth, as either the only path to the fullness of truth or even the fullness of truth itself. His insistence is not necessarily annoying and offensive, though it often is, and it is not necessarily explicit. He may simply assert that decency demands the adoption of his extremism, and leave the conclusion regarding those who don't adopt it unspoken.
I agree with Steven Riddle that extremists, even scolds, are useful to society. I'd guess, though, that among extremists, only witnesses are good in themselves, and then only when what they witness to is actually true.