If you run the wipers when they are frozen you might not burn the motor out, but you can cause the blades to jump off the screw pivot. Our Toyota Highlander had this happen last month--didn't realize the blades were frozen, and when I flipped the toggle one blade came free while the other stayed in place. Fortunately I did not strip the screw that held the nut that secured the blade, and could just retighten it, but it was not enough and my fix failed at a bad time (later while the vehicle was in motion). The dealership fixed it for free when I asked them for the proper setting for my torque wrench so I could do it. Lesson learned.