Well yeah, but if he's the only guy in his group then he may essentially "order himself" to disembark and fight, no?
If a unit is in a vehicle, he will try to drive on (if the vehicle is unarmed), or will engage with the vehicle's weapons.
Unless he is given a specific order by a player commander, or through scripting (for example, using a HIT eventHandler to detect when the unit is under attack, and then ordering the unit out), which overrides the default behaviour. I have never seen the driver of an unarmed vehicle stop and disembark to engage the enemy of his own volition when he comes under attack. He usually just drives on, and gets shot through the windscreen...
But maybe I just don't spend enough time playing with unarmed vehicles
Certainly, when the driver of a repair/ rearm truck or M113 ambulance comes under attack when on a support mission, they just keep driving towards the group they are on their way to support.
-PsyWarrior