Oh, and I got the impression that Eladrin was being a bit snide with the comment about 'everyone loves bats and rats'. That's the whole point of the "at least more than slimes and oozes", which are tied for least favorite thing in DDO in the lastest feedback thread (air elementals being the other one).
So I think they are intending to have nuisance creatures in the catacombs, but decided that bats and rats are better than slimes and oozes for that role.