I wear the Scropion EXO 900, which can be transformed into a three-quarters open face helmet. While I like that feature for the 85+ hot summer days, I don't know that I will be using it because of the limited protection in the event of a crash - plus it is no good if you get caught in rain.
Speaking to the benefits of a full face helmet, a buddy of mine was seriously injured a couple weeks ago (broken toes, leg and hip) after a crash with a car at an intersection. No alcohol was involved, and he says he was going less than 20 mph. His full face helmet is cracked in the chin area, and there is blood on the face shield. My guess is that he would have had serious facial and head injuries had he not been wearing a
full face helmet.
Another friend of mine, who wears an open face helmet, suffered a broken tooth and serveral other facial injuries when he had a dizzy spell and fell off his bike while stopped.
So much for the open face helmet, I just won't ride when it gets real hot.