"Super nice" stuff is not the only stuff with sponges... Lets see if I can make you a SHORT list of the 1000s of things found in LR.
Worms of all types (Peanut, Bristle, Polycreatic, filter feeding tube worms, Medusa worms, etc...)
Micro fauna and algae
Sponges
Bacteria variety
Micro and Macro crustaceans (Rotifers, Cope and anthropods, mysis and mysid shrimp, micro starfish, and the list goes on)
I could go on for a while.... All the above thing contribute to a balanced and long thriving ecosystem in your tank. IMHO, you can not have a successful tank without them. These things just do not come out of thin air and all can not be introduced into an aquarium through "seeding". Many experts agree that the "cycle" of a tank is not only the ammonia cycle but the larger cycle of balancing the ecosystem using these micro organisms found in live rock.
Sure, with live rock you run the risk of getting something that you might not want, but you run that risk either way. I have seen dry rock that was dry for 10 years still give off hair algae once exposed to water again. Sure there is a risk of getting a bad type of crab, mantis shrimp, or nudibranch. If you are that worried about it, QT your rock before you put it in your tank.
I just think that you are starting off a tank with an uphill battle from the start if you use all dry rock to start it.
