A crab that is large enough to take a bite out of a CBB will need lots to eat. If it is not fed enough, it will probably die a slow death. If it is fed well, it will significantly add to the bio-load of a system.
I have at least two hitchhiker crabs in my BB 67g that I have not been able to trap. They are the main contributors to the huge amount of detritus we siphon out of that tank each week
IMO, the added bioload of a large, well-fed predatory crab in a sump is not worth it in terms of the nuisance algae it will eventually produce.