The fucntion of live rock is to give beneficial bacteria a place to colonize. It makes a good filter because of it's porosity, which is alot more surface area for bacteria than you would get with any other media. Therfore, it's used as a means to break down the nitrogen cycle. Ammonia to nitrite, nitrite to nitrates. But to lower nitrates to gases that escape our systems through surface agitation, you'll need an anerobic area for bacteria to colonize. It's this bacteria in the anerobic zones that will convert the nitrates. The aerobic bacteria on the outer surface of live rock will not do this. With enough live rock, there may be enough anerobic zones deep in the rock to sufficiently lower nitrates, but you'll most likely need more than the minimum 1lb-1.5lb per gallon.
Adding new live rock can cause ammonia/nitrite/nitrate spikes, but this is due to the die off that occurs when live rock is shipped or transported, out of water, ect. ect. If it's fully cured than die off would be minimal, but if you're mail ordering the stuff, expect some die off to occur. This is why it's important to cure the live rock when you recieve it. but as for adding to the bioload, the rock itself does not add anything, just the micro life inhabiting the rock, and the bioload from that is not enough to worry about, just the die off of that same microlife.
