Bushmaster, you want to know why you should wear a helmet? Here's a reason:
I went biking (on a mountain bike, but not on single track) on some of the dirt roads on the Army post to which I was assigned one Saturday morning a few years ago. Figured I'd have a good time and get a good workout at the same time. I never wore a helmet before--it looked too dorky. However, post regulations stated that if you were on a bicycle, you wore a helmet, so I did. Long story short, near the end of my ride I started down a part of the road coming off a hill; it was probably 1/3 as steep as the one in your picture. Half way down I hit a small chuckhole with my front tire that I couldn't see due to shadows from the morning sun. My tire immediately turned, stopping the bike, while I continued over the handlebars. My air time could probably be measured in fractions of a second, but in that period of time I first landed on the top/back of my head, then rolled into the fall on my back, which broke between the 10th and 11th thoracic vertebrae, crushing my spinal cord. Result? I went from barrel-chested, ass-kicking, freedom fighting Special Forces trooper to lifelong paraplegic in an instant. The helmet moral is that if I had not been wearing the helmet, I would have gone from barrel-chested, ass-kicking, freedom fighting Special Forces trooper to corpse in the same amount of time. I still have the helmet--the plastic shell is only scuffed, but the styrofoam under it where I hit is compressed over an inch. Want to guess what that kind of force would do to a thin, flat portion of bone? You never, never know when you'll get into an accident, so you can't say, "Well, I wasn't doing anything dangerous, so I didn't need protection." Do you only wear a seatbelt when you're doing 125mph on a sinuous, wet stretch of road? I was out for a simple bike ride on established dirt roads, nothing nearly as dangerous as what your pictures show, and look at me in my wheelchair now. Some may say they'd rather not wear the helmet and be dead than be stuck in a chair, but my wife and two boys are pretty happy I had the brain bucket on that day.
Mountain bike injuries aren't always to the neck. The rehab hospital where I recovered had spinal cord injuries on the third floor, and brain injuries on the second floor. Whenever I felt sad about being a para, I took a trip down the 2d floor halls and saw the poor folks with TBIs, unable to care for themselves, communicate, remember, walk, use a bathroom, etc. I lost everything from the waist down, but they lost their very being, who they were before the accident, their ability to think and reason. Which would you prefer?
I'm not trying to crusade a movement to require helmets--people have to play by big boy rules and make their own decisions. However, I think people should have all the information before they make that decision. If you want to continue to go without a helmet, no skin off my nose. I just hope that someone out there reading this says, "Wow, the consequences aren't worth me looking cool," and when they have a wreck, that helmet saves their life.