See my thinking is if I lose my passport I can get another when i get home although likely will be a pain, BUT if I lose my drivers license I can't drive when I get home which is critical to (1) get my dog from boarding; (2) get my kid to school;(3) get to work. I dont drink - at all - so my license pretty much just stays in my car. I know it's not the safe way to travel and I swear every trip I'll remember to take ID and insurance cards from the car glove box but alas I get to the airport with nothing but passport and luggage. Maybe this will be the year I remember.
Oh and we carry passports in case DD needs to be identified in any way - so since we bring hers we just bring all of ours and it serves as my form of ID too.