The only thing a full face helmet does is protect your chin area from scratches and possibly breaks. I have been riding for eight years. I have never myself, or anyone I know or saw, got injured or even had impact around the jaw. A mouth guard is a better protection investment that a full face helmet. I use it once in a while when doing sketchy jibs. Especially after my friend pulled the bar up into my teeth one time.
Im all for having fun, showing your tits and getting fucked up on a river float but it can be done without being a pig.