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.