I'm sure the better jobs are on the bigger ships so you really only see the cream of the crop. On the smaller ships they are probably new in their career or basically just aren't that good or they'd be promoted. Just my thoughts.
But honestly, other than opening the shows, I really don''t see them or care who's the CD. But, with that said, they probably do make a difference behind the scenes. Like the Captain, they set the tone of the cruise. A good Captain or CD probably also have a good crew. That's where the difference is to me.