i don't have enough live rock in my sump to speak from first hand experience, but i can say, you will need to "dust it off" every so often, because it will develop a layer of detritus, which eventually could lead to problems.
i suppose if kept after a sponge or filter sock, it should be in a better position to aid in filtration.
the benefit of live rock in the sump would be the same as in the main display.
one local fish store i know of has massive bins of live rock- that is not for sale, that they use along with a massive skimmer, as their complete filtration system. seems to work well. they have some really nice tanks and livestock.
the owner boasts that he can dump a gallon of ammonia in his water and it won't effect anything. 8O i wouldn't want to try that though.