So why pay for a passport for a cruise? For alot of years I felt the same. Even though the majority of the travel industry disagrees with me, I'm still not sure it's something you have to incur the cost of. My family (5 of us, no small expense) decided with our last cruise to go ahead and get them but more for the conveinience and the option to use them in the future. The total including pictures was approximately $700 give or take a few dollars.
Yes, it is true, if something were to happen and you need to catch up to the ship or return home from a cruise unexpectedly you would have a very hard time getting through customs without one. But I always wonder, what if you don't have the means to do so anyway? A passport isn't going to help you afford that $2000 next day flight. In the event that there is trouble in a port and you have to locate a US Embassy, it helps to have a passport but I would never carry mine off the ship. I simply carry the number for them to look it up.
I think having a passport is a good thing, just maybe not if it is at the expense of not being able to take a cruise.