Asked tonight at the Soccer parents meeting if we should require the kids to wear them.
Here's the 1st part: They won't wear them unless they really fit well.
There are several papers on the use of mouthguards – but I can't find anything that is specific to this question: What is the incidence of dental injuries in soccer (Football in non-US papers) players – and is that number significantly reduced in players who wear mouthguards?
Mouthguards are worn for several reasons, one of which is to protect teeth. Probably more important in most sports is to protect the brain. With a mouthguard in place forces applied to the lower jaw are diminished before reaching the brain. This is why boxers wear a mouthguard and why they are required in highschool hockey even though the players wear a full face guard that protects their teeth.
Since soccer often results in falls to the ground and knees and elbows to the face, I usually recommend the mouth guard to protect the brain rather than the teeth.
As I remember from playing (American) football, the mouthguard does interfere with breathing — a real problem for soccer players, who don’t get to stand around for 90% of the game…