It is a situation where being physically or mentally unable to do your job properly will get someone else killed.
If those who have signed up for the wrong job aren't removed or forced to shape up, people will die.

Very true. From everything I've seen and read, a carrier flight deck is an extremely dangerous place. Anyone who isn't paying attention or doing their share can easily get themselves killed. Even worse, they can get other people killed.

While I can't speak from experience about carrier flight decks, I can from the perspective of being a paratrooper (airborne infantry). The weak and the screw ups must be culled before they get themselves and others killed.