Charles,
I have never had any problems either. I think they are talking about ridged frame cars with out suspension.
I have a front engine dragster that is ridged, so with that i have special blocks of wood with rubber that I sandwich between the floor and the rear motor mount plate, and the floor and the front motor mounts. Then I run a strap over the motor from side to side and tie it down so it cannot move. Never had any problems with the frame transporting it like that.
I can see where a ridged frame that is oh too light, with the chassis supporting extra weight, could flex enough to break if only held in place by tire straps. To much weight gets bouncing up and down while towing, and stresses the frame.
Some common sense goes along way when transporting vehicles.
Tom G.