From my experience pastic pants cut down the smell by a huge amount, and catch almost all leaks if you think your diaper will leak. Depending on the material they can also reduce or increase the crinkle.
plastic pants are of course necessary over cloth diapers. (it's surprising how often we read about people that didn't realize that and tried wetting their cloth diapers without cover!) I've only seen plastic shelled disposables leak high in back or under my butt cheeks in back on the outside, so I'm a littke skeptica on how well plastic pants would have helped, seeing as that's right about where they end. Maybe I just wear disposables that are a little large on me? dunno. Not interested in plastic covers.