I carry my passport for every trip, then leave it in my hotel. Thinking is that if I get robbed/lose my drivers license, I can still board the plane for the return trip home.
Passport card is made for you! They're not officially accepted for enough things in USA that losing it is a major stress, it doesn't have your address on the front, and big plus: foreign countries who want to see a "national id" (because they have one and think this is normal) who don't accept a passport will happily accept a passport card (source: my own experience living in South Asia)