In addition to protecting a player's facial structure, and expensive orthodontic work, a mask could possibly prevent a concussion. If you have never read about the long term after-affects of teens with traumatic head injuries, it's worth doing the research. Much different than head injuries in young children or adults.
I am a believer in pitchers and corner infielders wearing masks because it will protect them from broken bones and serious facial trauma. However, a mask does not prevent a concussion. A concussion occurs because the force of an impact on the head causes the brain to shift within the skull. No matter how well padded, a face mask will not prevent that sort of injury, just as the most advanced football helmets available do not prevent concussions. Please do not spread the false notion that face masks can do things that they are not designed to do.