A washer on the studs behind the wheel won't work either. The wheels are designed with a pad just outboard of the bolt circle and the holes themselves are slightly outboard of those pads. What that does is force the wheel center to be flattened a bit as the lugs are tightened, which serves as a sort of lock washer function. Washers take that action away, and then the lugs are loaded in shear without much additional strength from the friction of the wheel center to the hub.
Mark has made the right decision. Wheels are not the place to compromise on safety.
