Coming off of an 11,000 mile road trip (7k out into the Rockies and back, 4k to New England and back) the only place where a truck tent made more sense than a ground tent was when I was bivi'ing out in a former quarry in the Black Hills of SoDak - the ground was awful, jumbled rock, and there were a lot of cattle coming through. That's it.
Everywhere else, it was way more convenient just to have a ground tent, or bivi inside the double cab of my Tacoma, or open air bivi in the bed.
With a ground tent, in places where I was going to be staying a while, I had a 1-man tent I set up in a NF, sometimes for weeks at a time, and I'd just take my truck into the NP or into town on a daily basis, without the need to break down camp. 2 tents, 2 sleeping bags, 2 sleeping pads: a frontcountry setup that stayed setup longer, and a backcountry setup for backpacking/climbing. The heavier, more robust set stayed in the frontcountry at the unimproved NF site; the lighter weight set would go into the backcountry of the NP. After 4 days or so in the backcountry, I'd come back to my frontcountry site in the NF, and my tent and sleeping stuff was already set up and ready for a hard crash.
At rest stops along an interstate or highway, or at pullouts in NPs, you can't set up a truck tent, so it doesn't do you any good there. Other places where you could set it up, if you need to go somewhere in your truck, you have to break down the tent, and store whatever else was in the bed of the truck ... then drive.
For people who are just going to a campsite, and remaining fairly static, or that particular campsite/trailhead is the foot launch point for all the other stuff, ok, a truck tent would make sense.