We have passport cards and passports. We got our cards in 2008, and they were good for 10 years, just like the passport. However, the numbers are not the same: one set of numbers for each document and different expiration dates. They are, apparently, not linked---except somewhere deep in government documents land. We like having it for easy slipping into the pocket when going ashore, and we use it for I.D. a lot of times. Obviously, we didn't need both, but we do use both. (As for needing to fly back because some is ill, all U.S. citizens have "right of return," regardless of documentation. They can delay you, but they can't keep you out. In the case of an emergency from a ship/out of country accident/etc., I can't imagine there would be a problem--evacuation people would take care of that.)