just seen this thread, and thought i would have my 2 penneth worth.
it is a good idea for slow moving, or wide/long loads, but as said if you are travelling faster than its not neccessary.
it is one of my pet hates to see trucks, or recovery flat bed type wagons flying down the motorway with all lights blazzing away, and more so these days as they are usually L.E.D. lights.
its not needed, once the vehicle is on the back, you are classed as a normal vehicle, switch them off! i think its an image thing! look at me!!!

(and i have experience in this field as im ex REME recovery sergent.)
cheers
shaun