For me, the main reason to branch out to other cruise lines has been the desire to try new itineraries not offered by Carnival.
While it's still nice to do the occasional laid back Caribbean cruise (which Carnival does well), during the last few years I've become much more interested in trying new itineraries and new destinations. Now it's become a personal goal of mine to try as many new cruise lines and destinations as possible.
I make sure that every cruise I take is either on a cruise line I've never sailed on before, or to a destination I've never been to before. That's where I get my value. Interestingly, some of these cruises on "better" cruise lines have been cheaper than Carnival.