Everybody thinking like the author should watch this video first.
What on earth does some jerk goofing off on a skateboard have to do with normal travel at low speeds on flat roads in good weather by a competent cyclist?
If you're doing tricks on gravel, or travelling on mud or snow, or goring really fast, you may fall, sure, wear a helmet.
I've been cycling for 22 years. I've smashed my head on the ground 5 times now.
NEVER was it my fault. Just because you're not a dick doesn't mean someone else isn't.
I've been cycling on roads for 25 years and have never hit my head on the ground.
It shows the benefits of wearing it. The guy in the video walks away totally unharmed. This kind of blow to the head can happen easily while riding a bike, specially in a city.