Given the idiocy of some airline and most TSA employees, not to mention the certain non-recognization of certain state ID's on other parts of the USA (try buying booze in Massachusetts with a non-MA DL), I would view this country as not one country but 50-plus countries (the plus being the District of Columbia, Puerto Rico, etc.) and always carry a passport when leaving one's home area code.

Besides, given the mental/chemical instability of the current administration, you may want to have your passport in hand if you have to flee the country for your safety in the event of a declaration of martial law, etc.