My understanding has always been that in most ports there is a fairly large charge to the cruise ships for overnight stays. It is probable that since 11 hours seems the max, that after that the charge substantially increases. It's probably the main reason they stop at numerous ports for a short time. Also of course, people aren't spending money onboard or in the casinos when in port!! I have been told it's one of the reason the 7 day cruise to bermuda is expensive compared to other 7 day cruises, especially since the Horizon and Zenith are the two oldest ships, more expensive to stay several days in port, people are spending on shore not on the ship, and the casinos are closed.