Agreed, I'm very fond of urchins as a rock cleaner. They will remove your coralline algae as stated but if you're running lights bright enough to keep SPS happy, you'll most likely not have much coralline either as they don't do well under super bright lighting anyways. I get some on the fringes of my lighting but not much at all. I actually prefer no coralline, they end up competing with my corals for my alk and Ca.