I find the hard covers collect all sorts of dirt inside them so this dirt is pressed and ground against the Aluminum case. Not much hold these covers in place so once the corner of the cover gets cracked, the covers tend to slide. I've almost dropped a client's laptop because it slipped from the cracked hard cover when I picked it up.
Some (not all) hard covers on the display can be extremely difficult to remove. I now require the client to remove the hard cover from the display because I don't want to be responsible for damaging the display. If the client damages the display while removing the cover, then the client is responsible for paying for the resulting damage.