I beg to differ. Sure you get a wider view if you zoom all the way out, but consider for a moment a player like me, who is an expert FPS player, and when playing Battlefield or PS2 is constantly on a swivel. The difference being, in those games, no one is in 3rd person so the playing field is leveled to a degree, but even that being said, I am going to try PvP in first-person and see how it goes. Granted I'll be with my guild so I won't ever be alone, but I think even a group of 4 experienced FPS players (who are used to needing lightning-fast reflexes) could wreak havoc on a group of 4 experienced MMO players (who are used to waiting for cooldowns and rotations) in third person.
We'll see, but there's no reason not to try, it'll be fun!
A really good example is the new Darkfall: You can be first-person, 3rd-person over-the-shoulder or 3rd-person zoomed out. People in first person regularly dominate those in third especially if they are using magic or archery. Melee is a bit different and 3rd-person people tend to win melee fights against first-personers.