Live Rock...Is there a point??

[Fraggle Rock2]

Recently I have been debating about what "type"of live rock would be a good choice for my aquarium in terms of filtration.

Of course, "researching" the web one would normally come to the conclusion that the best rock for filtration would be the most porous/least dense rock. So, perhaps something like "Fiji" rock.

After seeing some Haitian rock, not really known for it's porosity and hence denitrification abilities, I started to think about the actual "function" of live rock. I mean, it basically gets covered with all sorts of living things such as corraline algae and corals etc... So, how is it possible that the so-called biological filtration process can even occur?? Not to mention the fact that I really don't see how water can possibly pass through the rock at a rate high enough to be provide useful filtration, even not even being covered with algae??

So, is there any actual evidence to suggest that live rock does anything significant? Is the Haitain lettuce leaf rock getting a bad rap? It looks pretty nice to me, so is there any substantive evidence to back up my "educated" reasoning that filling a tank with this isn't the best choice?

Who decided that "live rock" actually contributed anything anyway?