My DD is a Senior in college and had never worn one. They gave one to each player at the beginning of the season. She tried it and has been using it ever since.
Funny enough, my DD doesn't use one while catching, but does while pitching. She says she clenches her teeth too hard when she pitches. I'll have to see about her leaving it in all the time.
The reason they help with concussions is to keep the top and bottom teeth from banging together and causing a secondary collision which usually does more harm than the first collision.
moving topic slightly, any reccomendations on mouth guards for those with braces that allow the player to communicate effectively. DD1 catches and plays GK for field hockey, and both require communication, the latter especially while action is ongoing (ie talking to defense, pointing out unmarked opponents, letting them know when she is playing the ball). The one the orthodontist sold us is hard to communicate with. For FH GK and softball catching, we are thinking of letting her use a type we have not seen before, that is basically just a thin layer of rubber/plastic for between upper and lower teeth, nothing to protect from incoming collision. seems targeted towards activities where athlete is already wearing some sort of mask, so more for teeth to teeth collision than something striking the face.