here's a quote i found that pretty much sums it up-
"I do not allow any of the free ranging species of crabs within my own aquarium system simply because they are all omnivores, capable of getting by on algae, but should they find that meatier items are easily available within your aquarium, such as snails, tube worms and a host of other inverts, they can become quite efficient little predators. The hermit crabs are often sold in great numbers as members of a clean up crew package. This may be fine for fish only systems that the crabs have little chance to be of harm, but within a reef aquarium that strives to keep a diversity of life, they can and will become very detrimental to the goal of maintaining any diversity of life. These concerns apply to all crabs, including the popular "emerald" crabs sold for algae control. The only crab species that I allow within my system are those that are commensals, never leaving their host coral or anemone."