It is surprising how vital things are when we don't have them anymore. That is when we really understand these sorts of things.
They are part of a normal, healthy coral reef ecosystem in the Pacific, and this makes them vital. They do eat corals, and thus make open areas for settlement of other coral larvae, aiding in diversity. In and of themselves, they do nothing "wrong."
They are overpopulated, pretty much due to various poorly understood (but potentially natural environmental changes or man made impacts), and primarily over harvesting of their primary predator for the shell trade. Hope they look good on someone's shelf.
But I assure you, were they to destroy them all, they would probably realize how vital they are. We know how vital the long spine urchin Diadema is to the Caribbean reefs now, because they were wiped out probably due to disease. They are recovering, but in their absence, much of the reef became dominated by algae and fast growing corals. We can look back through the fossil reef and see this has occured in the past as well...kind of an alternate stable state of the ecosystem.
We don't realize the impact of many animals until we're gone. Failing to appreciate the fact that we live on a very short scale but can impact much larger time scales is very risky, IMO.