In many states a "trailer" gets a permanent license plate for a couple hundred bucks one time fee. A motorized vehicle gets charged annually for plate fees based on weight and/or value. In michigan that is easily upwards of $500 to $1000, and why you see so many of these big rigs with Montana plates doing the remote llc thing.
I've yet to see someone with a 5er taking along a smart car. Not saying its not true, but I'd say rare. No need for a toad when you have your truck.
But I do agree. If I were using a Semi tractor for towing, then I may as well go the Class A route, because driving a tractor to the grocery store is a little over the top.