LR in a Canister?

I see no reason to use live rock rather than the sponges that come with it and possibly some other type of media.  The action we most want from live rock is to house microfauna like pods and to house anerobic bacteria deep inside it.  Neither of which is going to be done to any great degree in a canister filter.  The rock would work just like bioballs or some other media that has great surface area and hold denitrifying bacteria, but I see no use in wasting live rock for this over say bio balls.  Many reef aquarists, myself included, would in fact advise against using bioballs etc as the denitrificating bacteria in the tank with plenty of live rock should already be doing the job.  Jake likes to call these types of filters Nitrate factories since they do such a great job of denitrification, but we have so few things that remove Nitrates.