My sister and her husband use their passports for ID regardless of where they are going, even if just flying from Florida to South Carolina.
So do I, Dave. Living in the North, it gives me more options if my flight is cancelled, such as enabling me to fly through Toronto with no hassles. Another plus is that it indicates to the Agent that I am an experienced traveler, and I am often put in shorter security lines. The only negative is that I am not up on the latest passport rules, as I don't need to know the distinctions.