Since I am flying in the United system I never get asked by them for my passport. However American always asks for it when I travel through ORD, DFW, or LAX and Frontier has as well in DEN. Allegiant has never asked for it in Vegas yet. Though are just my experiences I have come across so far. I really don't know what would happen if you didn't have it with you. If it means you can only sit in the back I would be screwed with out it since I sit in the actual far more than I get a seat in the back. Loads between ORD, LAX, SFO, and LAS always suck when I am jumpseating!