I have actually discussed this with friends (we have a long drive).
We wondered if it would get foggy inside, or would rain and snow would stick to the outside? Sweat and spit could get all over the inside too. We thought about putting rain-x on the plastic shield, but then you have more to maintain. Open bars just seem easier to maintain.
Inside a closed helmet like that would be loud. Any noise the player made would reverberate back. Consequently, it would also seem to limit a substantial amount of surrounding noise.
But player recognition would be increased. Their mugs would be a lot more visible...