I love this board!!!!!
I guess about 20 years ago some german guy found that if you put a kick butt protien skimmer on a fish tank with nothing else for filtration at all, but had a LOT of live rock in there, you didn't really need anything else. They call this the 'berlin' method, you may have heard about it.
A lot has been learned in the past 20 years or so about live rock, but a lot more is still unknown. Geez, just look at the thread on deep sand beds on here!
From my own personal experience, and I have been doing this hobby since 1990, is that live rock makes a MUCH more stable system. The more live rock you have, the more stable it is. I have had hang on filters, protein skimmers, canister filters, sumps, UV
sterilizers and NOTHING makes the tank more stable than a ton of live rock in there.
Just my personal experience tells me that you should have it in your tank even if you never want to have corals. It has so much 'good' going for it - and nothing bad. The fish love it to eat off of or hide in too. I think the fish (most are 'reef' fish, right?) need it from a mental stand point as well.
I have kept a 2 1/2 gallon nano tank with nothing but live rock for filtration since 2004. just an empty hang on filter for water movement and a couple of pounds of live rock. You tell me what is 'filtering' the water in there? The hang-on has a sponge to catch big nasties but other than that, the rock is the filter. My nano corals breed like crazy and my red-head goby is fat and happy, and still alive, two years later. If it works in that little tank, it only makes sense to work in a larger more heavily loaded system too.
At this point, I am done asking "how" and "why". I just know that live rock is good. very good. You can't ever really have too much of it.