I'm not voting because your poll is a little too black and white. There are times it is a stupid idea, there are times it can be a great idea.
In the scenario you set up in your first post, a glove box gun, I don't really see the point unless you don't have a CCW and you live in a state that allows you to keep a loaded gun in the car (it used to be common that many states had laws allowing loaded guns in cars but no CCW laws, but now few states, possibly none, that allow loaded guns in cars don't have shall-issue or easy "may issue" CCW laws). If your CCW location is hard to get to while driving, have an alternative place for it while you are in the car.
I can see temporarily locking a gun in the car when you are CCWing your gun and go into someplace where you cannot carry. Though, I don't really see that as a truck gun, but rather as a temporary situation.
However, like a few others have posted, when I envision a truck gun (or trunk gun if you have a car) what comes to mind is a rifle or shotgun. At home, a pistol is to fight your way to the long gun. A long gun has many advantages over a pistol (better practical accuracy and most power being the biggest). Just because you are away from home doesn't change that. Sure, there are plenty of possible scenarios where you won't be able to get to the long-gun and it won't help you. However, if an attack happens while you are fairly near your car, the handgun could help you get to the car where you can get to the more effective long-gun.
Now, I don't like the idea of a handgun locked in my car. A car is easier for a thief to get in than a house, and at home, I keep my guns in a safe when I'm not around. While it wouldn't be my fault that a criminal decided to steal from me, I don't want to make it easier to arm him/her. That said, few long-guns are used in crimes since they are pretty much impossible to conceal. So, while they may steal it to sell, I'm not really making it easier for a criminal to get a gun that may be used in future crimes.