For me it's wasting time every time I leave the ship! I don't cruise for the ports, if I did it should be much cheaper to just book a trip to the portcities!
I have been on two Mediterranean cruises with the same itinerary, Barcelona-Marseilles-Genova-Naples-Messina-Valetta-Barcelona, and we are planning to book a third cruise with the same itinerary just because we love the ship! We plan to stay on the ship in Marseilles, Genova and Naples so that we can maximise our time on the ship! Messina and Valetta are worth to leave the ship for, we think.