On half helmets the vents do cool things a bit, and on full face helmets they work even better, cooling the forehead and scalp, making the faceshield less prone to fogging.
In the old days of the full face helmets, I would close the shield till it touched my thumb (leaving it cracked approx. 1/2" and that worked well for full face helmets w/o vents or "nose-breath guard" (velcro insert to keep nose and mouth way from shield).