Well they don't rainy, so I don't think that problem exists.
As for thoughts, I like the idea that Pokedex entries are not to be taken literally, there are so many problems with things in them. I like to think they are guides, or rough explanations or something along those lines.
As for being used as a shield, Shellder are typically reasonably small, so I can't imagine them being use for much more than a Buckler. They are of the same general weight (around 1-2 kgs). The problem is if they are as hard as they say, attaching something to enable holding the shield could be seriously difficult.
Assuming that problem is resolved, I don't see why you wouldn't use them as a small shield, good for protecting yourself against a number of things out there.
