In Alaska, we used to run Super Cubs with the doors open and DeHavilland Beavers with the side door off and things sticking out that would not completely fit in the cockpit, like fuel tanks and 4-wheelers.
I'm just not seeing the big deal here. It's not like they don't have more than enough O2 masks. And can't a 2nd side door be MEL'ed?
Slider... <sniff, sniff>... you stink.