Talk about helmets always incites vehement disagreement between folks that believe in their value and those that don't. My opinion? I haven't heard any arguments from the no-helmet crowd that make any sense, and my personal experience tells me differently.
At about age 16, I was thrown off a motorcycle at 45 mph and, after an absolutely graceful, parabolic trajectory, I landed on my head. I had a good helmet on and I was able to limp away (gashed leg, and I had back pain for ten years).
My father was broadsided at an intersection in Bakersfield, CA back about 1973. He was thrown from his motorcycle, about fifty feet or so, and landed on his head. His helmet (a Bell Shorty) saved his life, but he got a concussion that laid him up for a few weeks.
An acquaintance of mine, "Eric", was sitting in his driveway, on his shiny new Harley, polishing the gas tank. He lost his balance, fell over, and hit his head on the cement driveway (no helmet, as he wasn't going anywhere). He died 3 or 4 days later. You can suffer fatal head injuries in an impact as low as 4 mph.
A helmet is not a cure-all—no safety device is. I know that in any collision with a four-wheeler, I on my motorcycle am going to be the loser. What would be a "fender bender" for an auto is often death for a biker. We can easily lose a leg, have our organs destroyed, and/or suffer paralysis. We slide across asphalt grinding rocks, glass and who-knows-what-else into our flesh, all of which must then be scraped out by a big strong medical professional who uses sharp pointy objects and no anesthesia to dull the pain.
Since I love to ride, and cannot quit, I have a responsibility to protect myself as much as possible—I wear a good helmet (with both Federal Department of Transportation and Snell Foundation certification), eye protection, protective clothing, and good boots. Knowing what I know, not doing this just doesn't make sense.
Keep it between the fence-posts!