The problem with those hard cases is eventually some random dirt gets inside the case and sits there causing scratches. A co-worker has a red plastic hard case on her macbook, she took it off awhile back to replace the hard drive and was shocked to see that her macbook was filthy and all sorts of scratched up on the inside of it.
If anything I'd put a wrap on it like the one from Zagg, then put it in a case. That way the metal is still protected but the case will help if you accidentally drop it.