"Drop a colour television set on the floor. It might work afterwards... but it's never quite the same."
From a neurosurgeon I worked with a while ago who ran a Concussion Clinic, for people who just banged their heads, maybe didn't even lose consciousness. He saw a lot of people who were significantly but subtlely damaged by their head injury. They had long standing headaches, difficulty holding a train of thought (Just were did I get to in that column of numbers?), little things that aren't obvious from the outside but mean that you can no longer hold down your high performance, well paying job.
In my own work, 12 years as an intensive care specialist and specialist anaesthetist, and a further 9 years just in anaesthesia (anesthesiologist, not nurse anaesthetist, for those of you from the USA) I have seen many people with severe brain injury from many causes. Most bits of the human body heal reasonably well with good care but the cells in the brain and spinal cord don't.
This thread began with a description of a severe low speed head injury, from the description almost certainly caused by an extradural haematoma from a middle meningeal artery tear. Even going slowly, less than 10 mph, (less than 16 km/hr), or just an awkward fall while actually stopped, can generate enough force to produce a lethal brain injury from an extradural or other mechanisms. As the doctors and this guy's family clearly decided, there ARE fates worse than death.
Please pardon me if I'm preaching to the converted, but if you aren't wearing a helmet for any speed sport you need to think very hard about why.
You only get one brain, and brain transplants are not an option. A helmet will not make you indestructible, it will not necessarily save you if you smack into a tree or a lift pylon at 30 mph, but it gives you a bit more of a chance of surviving and having a life with some quality of existence.
How much is your brain worth to you?