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.
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.
I walk a lot on well traveled streets. I recently noticed an interesting correlation: cyclists that wear helmets tend to follow the rules and ones that do not tend to break them. Rules like stopping at a stop light, not riding on the sidewalk, riding on the correct side of the street.
It's not a perfect correlation, but it's very high.
I rather correlate helmet-wearing cyclists to be the "wilder" bikers - since they know that they'll drive dangerously, they better wear a helmet to (feel to?) reduce the most dangerous consequences.
Or to put it in other words: you are more inclined to buy a helmet if you are know that sooner or later you'll surely need its protection. Or since you have a helmet, you know that you can do things at a (psychologically) acceptable risk that you wouldn't do otherwise (the latter phenomenon is called "risk compensation").
One theory. People who wear helmets tend to be people who or less "into" biking and more insecure about biking and their biking skills. Thus they play it safe and default to following the rules. People without helmets tend be the ones who view themselves as better bikers and feel more secure in their biking skills and thus think the rules and more like guidelines and that they, with their superior skills, know better and can safely ignore them when they feel like it.
There are people who ride bikes because they love cycling (by choice) and there are people who ride bikes because they can't afford other means of transportion (by need). The latter tend not to wear helmets or follow traffic rules, the former tend to do at least one of them.
Personally I almost never wear a helmet but scrupulously follow the rules: they are the only thing that protects me.
I've only ever bashed my head on the ground at very low speeds. You'd be surprised the damage you can do falling from head height. A bike merely increases the probability of that happening.
High speed; forget it. I drive a car now after I was totalled by a taxi.