Quite agree Jonty,
My company neither pay me enough, or train me, to risk my ass to assist a wheelchair passenger out of a burning wreck. I will have probably gone out the flight deck windows during the moment of evacuation (flight deck door being blocked by the evacuating passengers). So for any flight crew to re-enter the aircraft would mean trying to get back up the slide (somehow). Not really going to happen is it? In most cases cabin crew would not be able to lift the passenger to safety, and in some cases would not even try. A 20 stone man is a bit much for anyone to lift (unless you are a welsh weightlifter). Human instict would be to get out and leave it to someone else, such as the fire crew, if they happen to be there in time. Otherwise they become what is known as 'Fire fodder' (horrible expression, I know).
It really is case for the 'greatest good for the greatest number'.