A lot of players like to keep things fairly realistic, and disable 3rd person. However there are some instances when it would help to be allowed specifically for vehicle driving.
When maneuvering a truck, particularly in a base or crowded area, it can be impossible to see outside the cab area. In real life you can at least lean out the windows, or check the mirrors, but in game this is not possible.
Enabling 3rd person view purely for drivers only would allow easier driving, saving accidents like flattening friendlies or getting stuck on objects, and would keep it disabled for ordinary infantry and passengers the rest of the time.