There is a great post on www.girlsplaysports.com called Mouth Guards are a Must... It is a must read of parents of all athletes boys and girls.
The Main Point
I need to get my daughter to wear a mouth guard. She has had 2 bloody lips since the AAU season started 3 months ago. Luckily she has not lost any teeth. She has also taken a charge and fell back and hit her head. Read Youth Basketball: Busted Lip
According to the post on www.GirlsplaySports.com mouth guards are proven to reduce the risk of concussions too.
I have seen some kids wear mouth guards for basketball and soccer. I would guess about 2 out of 10 at the most. If my daughter started wearing one, she be the only one on her current basketball team wearing one. No one on her soccer team wears one this year either, although two girls did last year.
She is proud and will resist wearing a mouth piece until someone else wears one. She will need to convince her best friend to do it with her.