Now, you'll notice in FBAO that most-all of the monsters are slimes. They've got bunny slimes, centaur slimes, dragon slimes, bunny centaur slimes, and such. This was Isabel's idea, and we just kinda went with it. I think it works better than rats for the noob dungeons and scales up nicely.
We both have different reasons for doing this - on one hand it's a parody of the various slime monsters you see in Dragon Warrior, or Maple Story, or such. My reason is that I don't like killing humanoid things for no good reason. When I played Pikmin, if I lost a large number of 'min, I'd usually reset instead of exiting, gathering more, and starting over. When I played StarCraft, I played the Human side very cautiously, slowly advancing my range of control until I could take out the enemy base in one go. (Contrarily, when I played the zerg, I'd think nothing of sending 40 zerglings charging into the enemy cannons if I got bored).
I know that narrative constraints prevent MMORPG players from making major, permanent changes to the landscape, but I think that you should be able to at least pretend that you're the hero of the story (not just an adventurer or a guy-who-kills-stuff). As a hero, it doesn't make sense for you to just go to where the orcs are, and just charge in swinging a sword. They're just hanging out in a cave. You should have some kind of reason for wiping them out. That was one of the reasons I couldn't get into World of Warcraft - you've got a bunch of Kobolds who are just sitting around outside of town, holding a peace march or something. If I'm going to go kill 20 of them, I want a better reason than "Bjorn said he'd give me new boots".
So why do you kill slimes? Well, since they're all one thing, we could come up with some kind of backstory. But they're not just a separate species with its own culture and customs. It's slime. It's not supposed to be there, so you remove it.
It also answers the question of where it keeps coming from. It re-forms or buds or what have you. That way we don't have to explain why there's a town of 50 orcs, and 400 adult orcs get killed every day. Some areas are purified, so that slimes cannot enter. Towns, for instance, are permanently purified. Perhaps some player-types could temporarily purify areas. That would probably be a cleric's area-attack spell. It'd do a certain amount of damage per second or prevent slimes from regenerating HP or such.