They've gone over their reasons a few times. It's a pain for all the camera work and such, among other things.I personally would love to have both available as an option. I don't see why you would only want one or the other especially if you are playing the single player mode or on lan / private server with a few personal friends. That just feels like limiting your options for no reason. Just keep the official servers legit first person only for the hardcore pvp crowd and allow a toggle for Single player and private servers. Best of both worlds IMO.
I'm pretty sure that they've said that the point is not this, though.the point of the game is for it to be as realistic as possible adding 3rd person in any aspect changes that