20 vermin RHD, little ability to use equipment, mindless, and one of the few tricks it does have requires it to pay hundreds of thousands of GP. Even if you manage to play this pre-epic, it's not going to be contributing much.
That said, the crab isn't as simple as many of the other brute monsters that've been reviewed here. The crab's magic shell (mechanically more of a magic item every siege crab has), gives it some pretty big defensive bonuses, and also protects a hollow compartment inside of it. This compartment can hold a small number of creatures or cargo, which actually enjoy quite respectable levels of protection. It doesn't make them good, but it does make them interesting enough to use as part of an encounter sometime.
Bad monster, but cool abilities, and probably best used as a disposable exoskeleton for the party caster. -0 LA. Same goes for the 30 RHD Greater version.