When you think of it, it was stupid devising a system whereby a captain could be denied access to his flight deck.The one key person you want in charge in an emergency,doesnt have access to all of his aircraft.
The alleged reason for giving no crew member an absolute door lock override from the cabin side is because the code, card, magic wand, whatever, could be obtained under duress by an hijacker. We can question that logic but to the designers of the system it probably seemed eminently sensible.
I remember a briefing on use of the security door. We were told in no uncertain terms that the door must not be opened for any reason in the event of a hijacking, even if the hijackers were murdering passengers one-by-one to force the door to be opened. No heroics, no negotiations. Remember, the armoured door was never designed to save the lives of passengers or even to stop hijackers destroying the aircraft. The purpose of the door was purely to prevent hijackers accessing the cockpit and using the aircraft as a missile.