Helmet AND seatbelt can be dangerous, unless you use a HANS device, so I can understand that if you use one you shouldn't use the other. I think I already mentioned that BMW specifically asked for a regulation change accross Europe so that the C1 bike could be used without helmet. The UK refused and BMW almost didn't sell that bike up there.
A major cause of death amongst drivers during races was through violent head movements, where the body remains in place because of the seat belts but the momentum keeps the head moving forwards, causing a Basilar skull fracture resulting in serious injury or immediate death.
Of course the M3W seatbelt is not as efficient at restraining a body as a "race" seatbelt, but anyway, for continued use in countries where the law demands Helmet AND seatbelt to be used simultaneously, I'd suggest M3W drivers to use one of these:
