If a snail flips over and dies, it will raise your nitrate levels=bad for tank. Helpful hermit cleans up the poor, helpless snail=good- taking care of waste.
Think of it this way- by having to add a few snails once in a while you get to make new friends and see different kinds of snails.
Now, if your crabs start to like what they're doing and picking off snails that are otherwise doing their job and can keep their balance, that's another issue and I would remedy that.