I didn't want to completely clean up and leave for sight seeing, someone would probably take the campsite inadvertently. I did take the camper off once, what a pain that is.
A lot of people use the slide ins because they like to camp off-road in the boonies. A guy I worked with had one because he liked to pull his boat behind the camper and the truck. There are advantages. No separate tags and taxes.
I think there are a lot of people that have the same dilemma I had, that is why they pull little cars behind their big motorhomes.
Some people pitch a cheap tent to mark their site as taken. I left my shower tent and chained my 30lb lp tank to the picnic table at one free camp that had electric to show it was in use. It's by no means a guarantee but most people won't take a site they can see is occupied. It was vacant when I got back so in theory it worked. I can't remember if anyone new was there in the campground or not.
Some states require a license plate for slide ins if you reside there.
The problem with towing a car behind a MH is then I'm towing something again, and twice the insurance on top of half the mpg. I might as well get a small 15-20' travel trailer and still get double digit hwy mpg with this IDI. If I was traveling with a partner/spouse full time who would help pay for everything, then it might make sense for a MH/car setup.