Most tanks do just fine but when you get to tanks with working hull mgs for japan the problem with the strict system is apparent.
Currrently the priorities are
1 or 2 driver and gunner
3 hull mg
4 loader
5 commander
Normaly this isnt a problem when you spawn with a full crew but if you dont have all slots filled yet or took losses somehow a seat is going to be empty. But sadly the hull mg must always be manned and you cant have a loader instead.
But here is where the custom priority could come in handy.
If you prefer having a commander over a hull mg you could just swap the mg with the commander on priority 5.
If you need the mg for your playstyle and actualy find it more usefull they you could just keep it at priority 3.
But the current implementation of the hull mg (heavily restricted field of fire, low ammo mags and tanker stuck in the slot) makes hull mgs more of a liability for the tank than a blessing.
So please allow players to adjust priority for those slots or make AI use the hull mg for close enemys (like 30m in front of it in its LOS) when not in use by the player on tanks with playable hull mgs.