I'm surprised with some of the experiences above. I've only ever been asked for my passport on a domestic leg when checking a bag. The GA at origin has never once asked for it.
I've tried to initiate the doc check at origin a few times. Half the time, I'm turned away with "you'll do that during your connection".
Of course, I am asked for my passport on the int'l flight every time.
In the past six months or so, I have presented my PP at check-in getting boarding passes but was still required to show it at the gate to board the domestic portion of the trip. Swiped at BOTH locations. And to visually show it again at INTL boarding.