no it doesn't. most people only have a small section. In my sump for example. the water drips through filter wool, then through carbon and phosphate remover into a resouviour(sp!). From there it flows under a baffle and up through some live rock. here only the water can get through as the crap which wasn't stopped by the wool etc is too heavy and sits at the bottom. After flowng through over this top baffle it is sucked up by the protein skimmer and skimmed. from ber it is deemed "clean" (LoL) and sits in another resiviour waiting to be pumped back into my Tank....
So in short, no You don't neeed alot of live rock in your sump... if you have live rock in your tank.