Listened to some medical people discuss it one time.
The human head is heavy and not supported well. Like and orange on a tooth pick. Adding a heavy helmet can make it worse. The added torque from the weight can actually lessen the chance of survival.
Some helmets make it harder to see.
I guess the best time to wear a helmet is in town, at low speeds. Helps with a direct impact.
On the highway, a helmet does not really do much for you. If you dont get the chance to lay your bike down properly, a helmet is not going to save you.
Having said that, I always wear a helmet! Mine has a shaded lens and protects my eyes and face from 75 MPH grasshoppers.