Always wearing one under the helmet, myself. When it's cold they help, when it's hot it keeps the sweat out and sun from directly hitting the head, keeps any bugs away from my scalp, and since I wear glasses, when it's really raining they are a lifesaver - peak down and head down to coover the glasses and I can see. If I'm not wearing sunglasses they really help in the sun, too. Keeps the helmet from getting too manky from the sweat, as well.
Often I wear them backwards if the weather is nice so the peak doesn't obstruct the airflow.
Mostly I wash them every 4-5 rides or so.