Lines J. Hutter
Though I believe it's just the assault shuttle, not all shuttles.
Also - bear in mind if you stored them in the lift cars you'd have to check for ambushes down the corridors...
Sure, but I'd have to do that anyway once they leave the lift car again. But no check for Ambush in the lift car, so it's a safe place. And if they don't need a marine to hold their hands there it seems like the ideal place.
Actually if you play by the rules even crew members alone in safe zones will try to move to the next marine occupied space. At least I couldn't find it mentioned as an exception.
It says "casualties can safely be left unsupervised". But a healthy crew member is not a casualty.