I've ridden in hot weather both with and without (without does involve the use of a ball cap), and without is cooler. My hair is a little thin on top, so a sunburned pate is distinctly possible. Therefore, the cap.

Helmets DO, however, provide some ventilation, I've experienced it. Just not as much as WITHOUT.

Chris516, take the helmet safety argument elsewhere, there's plenty of space already devoted to it.