Here are some 1" - 3" diameter cores that I drilled out of Haiti (first two from left) and Fiji (right) live rock. The inside is quite dense, like concrete with only a few worm or urchin holes. I often find large black burrowing worms in the rock, about the size of a slug. This die off is just as bad as sponges when you add new live rock to your tank. This is one benefit to using artificial rock as there is no massive shock of die off. The addition of ammonium chloride daily will help speed up the establishment of a nitrogen cycle.
Man made rock is comprised mostly of marine calcareous aggregate with only 25% cement content. I don't see any significant difference in the surface area or void spaces in artificial or natural rock. Nitrifying bacteria live on detritus trapped in the substrate and in rock surfaces. There really is no shortage of nitrification even in bare bottom tanks with only a few rocks, manmade or natural. A lot of time was spent discussing how to increase porosity on man made rock in this thread, but very little time was focused on why we should do so.
While it's true that you can't improve on nature, I don't think it's fair to say nature has evolved to use nitrification as its sole design for rock formations, nor is it fair to say that bacteria have evolved to only accept the exact pore matrix of coral rock. In the end, man made rock is encrusted with worms, coraline algae, sponges, and coral polyps, so the surface porosity is identical to that of naturally formed live rock. We are comparing sea apples to sea apples here.
One person has argued that man made rock is more harmful to the environment than live rock due to the trucking across great distances. I can assure you that the journey from Fiji to the end user in North America uses far more fuel. I don't know anything about mining portland cement, but I doubt it is removed from as delicate an environment as live rock is.
People make their own rock for a variety of reasons, and cost is (or at least should be) in the minority. I make it because it looks better, fits better, hides plumbing, and has limitless possibilities. The tank matures quicker (nitrogen cycle, coraline algae & misc. invertebrates) if you mix some live rock with your artificial rock, but there is no reason why the "live" rock added has to be from the ocean. I still use natural live rock, but I wouldn't be disappointed if live rock harvesting was banned tomorrow. Once the rock is in your tank for six months, it's almost impossible to tell which ones are real and which are artificial.
