The truth is, it doesn't make as much difference as a lot of people think. Sometimes a helmet will save your head. Other times, it may save your head at the expense of your neck. Sometimes it doesn't make a bit of difference because you end up with exessive damage to some other part of your body. Since most people ride a bit faster than the 17.8 MPH impact failure point on the average helmet,anything more than an glancing blow or a collision with a solid object after sliding and rolling far enough to bleed off a lot of speed is as likely as not to embed sharp bits of broken lid in your skull.
Now, throw in the fact that you are somewhat more likely to get into a crash should a lid cause exessive distraction or fatigue or reduce your field of vision, and you will find that, in the long run, there is a very slight but very real advantage to not wearing a helmet under average riding conditions.
There are some conditions that favour wearing one. Dirt riding for example. On dirt trails, you won't usually be going any great speed, and you are sure to fall on your bum once in a while, usually into dirt or brush. Racing, you will probably crash into hay bails, other bikes going in the same general direction at about the same speed, or glance off walls. Also, you will most likely be wearing something to help guard against a broken neck.
