While I'm not a fan of most crabs in a reef system, I can say some crabs are good. Some of the hermit crabs are good in a reef tank. Other species are good in a live rock tank. Arrowhead crabs can be good here as they are interesting to watch.
I also think some crabs are good to eat, especially blue crabs, king crabs and Dungeness Crabws (grin)
Lisa B I'll go half way with You here, I love my blue leg hermits , they don't cause any probs that I can tell. I even had a tiny porcelain crab ( had to lookup the spelling ) very cool.I don't know where it came from or where it went. Now I recently picked up 2 hermits with black and white legs. I witnessed one assaulting a snail. It was quickly judged & sentenced to life in the refugium. The other one ended up in there to just because!
I like my red/blue leg hermit crabs so far. I like my emerald crab too, although he has knocked my zoanthid(sp) off my spot for it on my live rock twice now.
And yeah, crab cakes, crab soup, need I say more? lol.