I actually like wearing a helmet in the winter, but in the summer (Phoenix) I find that I sweat so much I can't keep the sweat out of my eyes. We went for a ride on the 4th about 8:30 pm to watch the fire works and it was still 105/106 outside.
I find the same to be true while dirt biking (Motorcycle). I've had a couple of mild heat strokes wearing a full face dirt bike helmet on longer desert rides. One needs to be careful and weigh the variables in extreme heat. Your mileage will vary.