I suppose that this may spark a debate about legal retention, but if you catch a soft crab please throw it back. If you catch a legal sized crab that's soft in the legs, there's virtually no meat on it and the little bit that is there is stringy and bland. Just give the back leg a little pinch, and if the shell goes in at all, then the crab has just molted and there's very little meat inside. If it's firm, then the crab is worth keeping.
I don't mean to start a long thread similar to the dark fish/throw it back ones that show up on here periodically, but it's the right and sensible thing to do. Put it back in the water and catch it again in a few months when the meat is firm and tasty!