Live rock, in some instances should in it's own right be considered an eco-system itself!! The reason I say this, is because there is a vast mutlitude of algae (many types from hair algae spores to coralline), worms, snails, small crustaceans, and even small frags of corals nestled in the pourous structures of live rocks. Not to mention copepods,barnacles, branchiopods, and if you are very lucky, and buy un-cured live rock, which was collected properly, I have known some people, including myself to actually have eggs attatched!!! Mine were stupid little yellow-tailed blue damsels (only 1 survived out of the few eggs that were there). I say stupid, because I already had like 10 of them, and I was all excited when I realised there were some eggs lodged in a hole----only to find out that they were the same species!!! GRRR!! --LOL, and yes I am sure they weren't from my fish, as it was uncured rock, and was in another tank while curing, where the other damsels were not, lol. But anyways, a vast community can reside within live rock, all of whom play important roles not only in the biological filtration processes, but also in the stability and diversity of your community ecology within your tank(s). Live rock is also the main foundation for any plant/"animal" life to grow -- especially corals!! Not only that, but it will soon proove to be wonderfull breeding grounds for your fish, crustaceans, and corals you already have. Either way you look at it - -Live rock is essential, and hey, to me, it can sometimes be as good as christmas morning, just knowing that in a short while, I may have things popping up from that rock that I never knew was in/on it!! Only downside, is that sometimes you get the bad with the good, and will have to remove some little beasties.
--EMI