I think the point of the headwear of any form is to make the unit a) look "uniform," and b) make them look TALLER. The difference between the WGI groups and DCI groups is the size of the venue. In WGI, there really is no need for headwear, because it is more of a personal, closer venue where seeing facial expressions adds more to the show. The audience at a DCI show is much further away, and typically outdoors. To me, a corps looks short and incomplete when they're not wearing a shako or aussie, or whichever headwear they choose.