Easy way to organize and set up PvP conflict within the context of the game world. Any other ways I can think of (all players attackable by anyone? PvE only zones and PvP zones scattered around the world? Random selection at the start of BGs?) are much more complicated than simply "There are two teams. Team A fights Team B." Those other solutions also lack convenient hooks you can use to tie the PvP content into the plot.

You could have a game with no PvP content, or limit it to duels and gladiator fights, but lots of players LIKE PvP, and Blizzard wants to take EVERYBODY'S money. Plus, PvP has the advantage of being a renewable resource in the way PvE is not. Every time you fight Deathwing, it's the same fight. He's not going to do anything to surprise you. But Battlegrounds have different 'mechanics' and 'spawns' and 'phases' every single time you do them, and all blizzard has to do is create a place for that to happen and turn the players loose. It keeps a lot of folks busy during content lulls.

So yeah, lots of reasons why mmos set up the "you guys vs. them guys" scenario.