Pros of one big server: Everyone can play with their friends, becuase its just one server, No RP servers required because you RP where you please, and those are both pretty big pros.

But with the way a lot of the game is designed, it might not work out well, because instances would become mandatory, and you'd never be able to escape that free company you hate or the jerks who offend you just by looking at them.
(although the latter is just a con of one server games)