I don't have a problem with dedicated truck or car guns but I don't have one for a couple different reasons. I don't drive the same vehicle every day and I'm not the only person who drives those vehicles either. My daughter and wife are just as likely to drive my personal truck as I am. But I don't think I'd ever use an AR, even some of the less expensive ones, as a truck gun. Lot's of other inexpensive options that I would take first. Hi-Point Carbines, shotguns, .22 rifles, lever actions, cheap bolt actions and any number of handguns suit my needs just fine.
Everybody's needs are different. A person wanting a gun to pop coyotes at a couple hundred yards isn't likely to have much use for a 12ga. A person wanting to take the back roads and hunt pheasants won't have much use for a Hi-Point. A person in the city doesn't have much use for a bolt action rifle. And so it goes.
As for defensive reasons to carry a long gun in your vehicle it really does little if any good. It's not handy in the vehicle, it's not on you when you're not in the vehicle. What good is it? None.
LK