I think that because the shell holds their internal organs, they die when it is cracked. It may be that the shell building secretions are able to fix the problem but even so a snail with no shell would be easy prey for birds and the like.
Thank you for that, Kimmiebear. I think you're right, it would depend on how badly the shell was damaged. The one I rescued the other day just had a small tear in his shell and he seemed quite happy.