Some cyclists wear a helmet because they think it can save them in case of an accident. Others are willing to take the risk and don't use it. However, there is a third type of cyclists, the ones that bother to buy and take a protective headwear, but instead of putting it on the top of their head, they hang it on the handlebars.
I noticed this phenomenon quite a while ago. At first, I thought it's mainly teenagers who try to keep their parents satisfied by taking the helmet, but want to keep the cool hair style. But then, on 28 March 2012, I saw this guy by the Royal canal near Leeson St. He was definitely not a child.