Not all urchins are bad - you just need to do a little research to see which are safe. Longspine (diadema sp.) are generally reef safe, but they can get very big, so they're not typically tank-friendly in the long scheme of things.
I forget the species name, but I have two of these guys in my tank now, and they're completely reef safe. They don't have really long spines, and they aso don't pick things up (just knock them down), and don't chase things down and eat them. From what I cal tell, it just eats coralline algae or the biofilm on rock.
