I plan to get a passport, even thought I have a valid copy of my birth certificate. This is because a passport can be used as identification that birth certificates can not, such as talking to police, traveling, getting a driver's license, opening a checking account, etc. There will be no question I am an American citizen anywhere in the world.
In addition, it is easy to renew a passport for the rest of my life, I can keep by BC safely tucked away and not risk damage or loss, and if a passport is lost or stolen, I can get it renewed at an US consulate. Like in the military, they have my information on record, too.
Just my thoughts.