Any player not wearing one for WHATEVER reason, will not play. It's THAT simple.
Act As a Barrier
Mouth guards are very important when it comes to any sport because they protect you from multiple risks that do not always solely include your teeth. When being hit in the mouth you could experience neck injury or even shock. A mouth guard acts as a barrier to keep you from having any problems.
Protect Your Teeth From Harm
When you are hit in the mouth hard enough without a mouth guard, your lower jaw can come in to quick and abrupt contact with your upper jaw. This can cause your lower jaw hinge to be forced in the base of your brain which, in turn, can cause a concussion.
Evenly Distribute Any Blow to the Mouth
Mouth guards should be 3- to 5-mm thick to reduce your risk of concussion. A thick mouth guard tremendously decreases this risk because any blow you take to the mouth will be evenly distributed among the mouth guard.