Yeah--but you can't always prove a disability. A while back my wife was injured on a trip. What proof should we supply? All we had was the doctor's evaluation (whether she even had a written copy I don't know--and if she did it wasn't in English) that flying home as scheduled wasn't going to exacerbate the injury.

It also occurs to me--there are those who only need a chair to board. They can walk but their endurance is limited, on arrival there are no security lines to slow things down.