It is important to remember that a cruise is two vacations in one - the ship and the destinations. A cruise line with just three ships needs to change destinations more often to entice customers back to the cruise line. Most ships, especially smaller, luxury ships are constantly repositioning so their loyal customers have a reason to cruise more often.
The best repositioning cruise is a world cruise, even if it isn't really a repositioning cruise since it ends where it started. Still, technically any cruise that sails from one cruising region to another is a repositioning cruise, so never mind the definitions, I'm on a world cruise. Ask me when I get back.