Its possible in real HMMWV so why not in game?

It would increase survivability of unarmed armored cars like hunter.
Imagine a guy with RPG aiming at you, you need a quick way to do a return fire and while you are still in vehicle you can fire while moving and part of your body is still protected by armor.

We already have possibility to fire from civilian vehicles and from trucks but who cares about civilian vehicles and trucks...

...While everybody cares about most popular vehicles in game like hunter.

Same thing with tanks and APCs, unbuttoned crew should be able to self defend using personal weapons.