TomR, a job is in it for them and in this state your lic# has to be visable on any vehicle engaged in work for the company. IE I drive my Pinto to the site, do a days work and drive home, I'm an employee. If I drive to the shop, pick up ladders, a sprayer and 30 gallons of paint, I am considered a commercial vehicle engaged in business and a license # has to be visible from 2 sides of the vehicle.
Pro, I,ve been using magnetic signage for more than 20 yrs. because it increases the resale value of your vehicles and allows you to go undercover to check out the competetion, also allow you to park in your driveway in certain neighborhoods. I have had one problem. A 1984 Blazer with silver metallic side panels, it had some staining but you couldn't tell that it was from the signs (no outline).
Put a few good coats of wax where the signs go and wash and wax the truck every few weeks, wash the sign too (both sides) and you shouldn't have any problems. I have done this with GM and Ford vans, pickups, Olds 88, Continental, a few Toyotas, Jeeps and a Nissan Maxima. NO damage, urban legend.