The only camera I ever used a case on was a Leica M6TTL in really sweet condition. That had a "Luigi" half case attached - which protected the base plate and back and, because it had a padded "grip" on the right-hand side, actually improved the ergonomics.
As I have never been someone who shoots roll after rool of film in the hope that I'll get a couple of good ones out of 100 shots (I know my photos are crap, so I might as well save the money) it never got in the way or slowed me down much.
I have a case on my Exa but it is removed whenever I want to shoot as the ones with a "front" are sharp, stabbing pain in the trouser region.....