American Driving Society shows require a bit in their rulebook. Although in my younger (more invincible) years I have rode bareback in a halter and a lead rope, I would never drive without a bit. When you drive, you have attached a "weapon" to that horse.
Also, more subtle communication can be achieved through the use of a bit vs. nose/face pressure, and that is exactly what a bit is for, Communication. Those that use the bit as some form of brakes and steering, like a car, misunderstand the use of the bit. Bits don't stop horses, horse's brains stop horses. Proper education of the horse's brain to understand that a bit of pressure on the mouth means I want the horse to stop, stops horses. With more proper education, it can also mean bend left or right, move laterally left or right, lift a shoulder, turn your head to avoid seeing that "horse eating rock" (with the proper use of blinders), lift your front end, etc.
I have to reeducate a young student I am working with because he seems to want to stop my little mare by pulling harder, instead of giving her a chance to hold the cart back. She sticks her nose on her chest with her mouth gaping, and the sheer fact that she stops is actually what is stopping the vehicle. What he needs to understand is that it is going to take Dolly some time to balance herself to stop that vehicle, and pulling harder just makes it more difficult for her to get her balance. Yup, Bit = Communication. Anything less is uneducated hands on the reins.
Myrna