My US passport isn’t expiring for a couple of years, but I’ve decided to renew it anyway. Here’s why:
As a non-EU spouse of a UK citizen living in France, I will be applying for an EU Family Permit and residency card in the UK to not only be able to visit the UK, but perhaps if we decide to settle there at a later time. This permit may take the form of a sticker or “vignette” in my passport. Were my passport to expire soon, I would then have to reapply and get my sticker restored in my new passport. So, why not start with a new passport that will outlive my need my permits?
…besides which, several countries require a passport be valid for at least 6 months from the date of entry in that country. Better safe than scrambling!