It's a lot easier to sustain head trauma than people think. When I was 17, I was clipped by a car while riding my 10-speed. I didn't have time to react. I ended up slamming my head and scraping my face on the pavement. I had a concussion, multiple contusions and abrasions. I was NOT wearing a helmet.
In addition, I was going about 5-7 miles per hour, which is very slow.
True, if you have time to react, you can hit the brakes and slide. Regardless of a helmet, you're still going to get road rash. But, what about those accidents in which you have no time to react?
In my opinion, it all comes down to which risks you are willing to take.