Those full face/protec helmets offer more protection especially for the back of the head where the roadie styles don't cover.
They are also safer because they look dorkier, there aren't any aer0 snag points on the rear of the helmet (when you rotate head on the ground it doesn't catch). When I crashed last year, I experienced that first hand.
Of course it pays off more to just pay extra attention to all the hazards on the road. Edit: For greatest protection I think you'd need to wear one of those suits that one guy developed for fighting bears. Or be Robocop.