Asheron's Call has monster play, too, but I don't think it's been terribly popular. It's kind of a hard tradeoff because it's not something players are prone to invest a lot of time in, but it still takes a lot to create and maintain development-wise.

Just think back to Wrath's Zombie Invasion event and how much havoc was caused by people becoming zombies. I thought it was great fun, but it seems the vast majority of players hated it.