I did not mistype my statement, dry rock does not become live rock without seeding it. There is not way to get the biodiversity on dry rock without seeding it with quality live rock. There is a lot more to live rock then coraline algae. If you want to use rock as a natural filtration, coraline algae on dry rock will do minimal for you. Also, dry rock that was once live rock will have dead organisms in the small cracks and crevices that you would never be able to fully clean out. This and coraline algae can seal the pores in the rock, reducing the biological filtration capacity.
Now with that said, back to mdub's questions, live rock is not necessarily required for a fish only tank, as long as you have some other filtration.