People with uncertain mobility might not know how long they need to start out in advance and whether they can stand in security lines, etc. Relatives might also pressure them to take a wheelchair out of concern that they miss the flight.

But if you have no connecting flight there's a lot less risk involved in getting off a flight by your own power, particularly when airlines forget wheel chair users.

Consider that before they board they are rushing around the airport and after they've just been sitting still and resting for over an hour.