It is all about your tolerance for risk - falls in the same category as "do I need
travel insurance" question. You are correct that they are not required - unless you have an emergency and have to return via alternative transportation. In this day and age, I would never leave the country without a passport. Yes, it is an additional expense, but the piece of mind to me is worth it. And for adults, they don't expire for 10 years, so if there is any chance you will have international travel in the future, get them now and you are set for awhile. That said, they are expensive. For all 5 of us I believe it was close to $500 by the time we were done.
Kevin