I have banned all forms of crabs in my tanks:

IMO (in my opinion) crabs=eco terrorists.
Disclaimer: not everyone believes/has had bad experiences with hermits, and my case may not be the norm.

I've had a 1.5 inch hermit kill a 5 inch. tang,
(he caught the tang in the back of tank at night in a very narrow area; tang was still alive when I found him in the morning, when I set him loose; he went shortly therafter).
1/4 inch blue legs eat feather dusters (I've witnessed this, wish I took pics),
emeralds strip live rock clean (seen this in tanks other than mine as well)

no more crabs for me

a.k.