I have always had issues with this particular law. I am pretty certain that if someone were to pursue this issue far enough in court court the law would get changed or modified. Most likely though the case would get thrown out and or dismissed before the law were ever modified.
I can see reasons why they thought this was a good idea (theft, unregistered offroad vehicles or boats that ARE getting used, hazardous/dangerous materials, etc...). But things on a trailer or in a truck bed are mine and what I do with them is no business of any one else so long as the truck and/or trailer are following all regulations and safety requirements. Registering a car, truck, or motorcycle for transport alone is just as ridiculous as registering a dresser, a bed (hey with coasters and a motor it could be considered a vehicle), or my groceries. Even intent to use said unregistered vehicle illegally is not a crime.