Exactly. Many people would agree it would be "better" to spend a lot of time in each city to fully absorb the culture. That's why my family typically spends time in the embarkation and/or debarkation city when we cruise in Europe. But that doesn't mean there is no value in visiting countries for just a short time.
On our first Med cruise, over the course of 7 days my 10 year old and 14 year old kids went to a Marc Chagall museum (with dozens of his works), the ruins at Herculaneum, a Galileo museum (with equipment from his time), the Uffizi Gallery, the Sistine Chapel, St. Peter's Basilica, the Pantheon, and the Coliseum. Did that give them a strong understanding of life in France and Italy? Of course not. Did that help them understand the context when things they saw were mentioned in later science, history, and art classes? Of course it did. Yes, we could have flown to Rome, stayed a few days, taken the train to Florence, stayed a few days, then gone on to see Naples and Nice on our own. But given the limited options for finding rooms with 3 beds in Barcelona, I didn't want to do that in 4 other cities too, and I didn't want to drag our luggage on trains, planes, and cars every few days. My husband and I also didn’t want to take off more than 2 weeks of work.
Disney Cruise did have a cruise scheduled to stop in Tunisia in June 2011, but the stop was changed to Sicily around February. That same cruise stopped in Corsica and Malta. I think there was a cruise stop scheduled for Turkey a few years that was changed several months in advance. I am not sure if Disney has plans to try again for the southern Mediterranean, but Malta, Sicily, Corsica, and Croatia have been showing up once or twice a year.