Well you have to keep in mind that aquatic animals are not that different from terrestrial ones. Yes you would think they would use a lighter touch when navigating rough terrain, but that is not always the case. What if the animal is startled and doesn't realize where it is until it slams against the rocks. This is especially true with almost every kind of cephalopod. When they startle, they BOLT! for it. They'll hit glass, rocks, sand bed, what ever is behind them until they slow down. Octopus are not as bad as squid and cuttlefish, but it still happens. You also have to think of the stress level if that animal has to constantly tip-toe around it's entire environment. As for fish and other vertebrates, they sometimes like to lay against, on top off, and behind the rocks. A lot of fish species even sleep inside holes and crevices in the rock. Jagged and abrasive rocks can and possibly will scratch and cut them. Now I'm not saying you can't use it. There are plenty of animals that would do just fine with those types of structures, and there are plenty of places out in the wild where animals do it every day. Like I said maybe if I were putting a reef tank together, but I would never put rocks like that in with any kind of cephalopod. Well unless I were keeping Nautilus Pompilius, but that's another story. lol