It is actually kind of simple really. If you have fixed identity (or even psuedonyms that are tied to a "true name" a la Chaumian credentials) then filtering out bad behavior becomes easier. Users cannot create sock puppets, they get one chance to troll and then join the global killfile or get put a sufficient number of user killfiles as to have the same effect. If the identity was non-pseudonymous or the pseudonym systems supported cross-domain joins it would get even harder to troll since a trolling attempt at site A might get you added to user filters at sites B and C.
The trolls would not go away, but no one would ever see them and this would have the same effect.
You might cut down on the trolling, but merely, say, cutting it in half isn't going to change anything.