There's another "insurance" aspect of having a passport that has nothing to do with cruising: a backup form of ID in case you lose your drivers license. This happened to me twice: once when I had my wallet stolen, and a second time when I moved between states and the new state confiscated my old DL, and issued me a temporary paper drivers license and sent me the permanent one in the mail a month later. In both cases, I had a temporary paper DL so I was legal to drive, but could not do anything that required a photo ID. A birth certificate is not a substitute for a photo ID, as it only establishes that the named person is a citizen, and not that the person possessing it is the named person (that's why you need a DL plus birth certificate for cruising if you're an adult).
But if you have a passport in your drawer at home, then you have a second form of universally-accepted photo ID. You can board airplanes, rent cars, check into hotels, and do banking all without a hassle while you wait for you replacement DL. In one of my cases above, the event happened 1 week before a long-planned trip to Hawaii. Had I not had a passport, it would have been more difficult to board the airplane, and impossible to rent a car once there.