The problem with emergence in MMOs is that once an emergent trick is found, it becomes an exploit, and then the emergent behavior has to be eliminated somehow, for the good of the game.

For example, in the MMO I am creating, you can catch bats, hold them for a while, and then they become your pets (if you don't hold them long enough, they remain wild). Some people gathered up hundreds of bats, then released them in a small room. When other players came into that room, they were swarmed and die. After letting the trap sit for a while, they would return, cast a fear spell to disperse the bats, and then collect the loot.

It was a cool trick, and probably super fun for the players involved. But, if I left the exploit in, it would be happening all the time, and would wreck the game for everyone else. In single player games, the worst a player can do is beat the game using an exploit, which is not necessarily bad.