While I do think that helmets do give some protection, I also think that wearing one makes me a worse rider who is more likely to get into an accident. I live in a hot tropical country near the equator. Compounding the discomfort by wearing a helmet makes it harder to concentrate, especially when you have sweat dripping into your eyes.
When I was in the army, we had this saying that everyone becomes stupid when they put on the helmet. It's not as bad, but there's a similar effect with bike helmets for me.