The problem with a rule about skill levels in aircraft is that skill is not easy to judge. Just because a person is not having a good day does not make them a bad gunner/pilot ...
We all have good and bad days, judging skill is almost impossible.
I do ask players to use assets wisely. If you know there is a better pilot/gunner in the game then let them have the aircraft etc.
I almost always defer to a better player.
Except when it's you!