Yes, your dealer was right. He was also, IMO, being a jerk!
I used magnetic lights on my Jeep for several years. To avoid the reported problems of the wiring flopping around on the side of the car, I installed the wire harness permanently INSIDE the Jeep. No problem, it can't flop around.
I never noticed any scratches on the side of the Jeep where the lights were stuck.
But then, that is one of the great things about towing a Jeep; It is a JEEP, it WILL get scratched and dented when offroading! It is the nature of the activity. So, who cares if the lights scratch the paint a little. Scraping by a pine tree or a clump of quaking aspen will scratch worse!