Urban - no. Too little need. Odds of it being stolen or other problems far exceed the reality of ever needing it, and even less likelihood of being able to get to it if you needed it.
In a car theft, carjacking, or burglary, you're just surrendering a rifle and ammo to a bad guy. In any type of public shooting, mall shooting, office building, school, etc. you're simply not going to be able to retrieve your truck gun and safely return/engage - and you could be mistaken as a bad guy...
I can think of very few reported situations in the perhaps billions of situations people with truck guns are faced with.... notably the Texas belltower shooter a half century ago when times were different. Civilians helped police put suppressive fire on the sniper to keep his head down and he stopped shooting. That's one incident in decades of a truck gun being useful.
You're much better served with positive control of a handgun and extra ammo on your person.
Road trips, extended journeys, and rural, and driving out of state - yes.
These are situations where needs can arise where you have a breakdown and need to be able to have more effective defense, or you're in an area where police response is a long time, or phone reception is poor, or out of state you cannot buy a second gun...
Even here, factor in what you will do with it if you have to abandon the vehicle... how will you discretely transport it?