Testing some crew return values in Arma Vanilla, I discovered that:
All killed Ai drivers give their place to another crew member, after few seconds.
There are some differences between vehicles while you're testing this on land vehicles, ships, helicopters or plane (V44 Blackfish). I let you discover the ballet.
But, it's absolutely inefficient. Generally, air assets has enough time to crash before the new improvised driver can react to a no-future trajectory.
And last but not least, all waypoints are lost despiste the remaining group (just one dead in crew)!
So, totally useless and even counterproductive, as far as a guy frees a gunner sit to stand behind the steering wheel / stick with no more objective/waypoint.