Maybe I should explain a little more. Many countries require a Visa for entry. Getting that visa when you have a passport is quite easy. Getting it without a passport is very difficult. They can actually keep you from LEAVING the country until you have secured a passport and valid visa to be there to begin with! Considering that you have to be in the US or at a US conulate to get a US passport, it gets very complicated getting one after the fact.
I believe Costa Rica, Nicaragua and Belize all fit into that category. Cuba is another story altogether (if you ever DO go there, do NOT let them stamp your US passport or permanently attach the visa unless you have specific permission from teh US government to go!).
I'm a US citizen with a Canadian Social Insurance card, as is DH. We still don't even enter Canada without our US passports. Having one just makes any number of emergency situations better.
Anne