The only good argument (in that article anyway) for not wearing a helmet seems to be the Bath university study, which everyone has been citing since the day it was released. But that's the only study I have seen on the topic. The fact that other modes of transportation (walking, driving) would theoretically be safer if you wore a helmet is irrelevant. And it's possible that helmets would turn someone off from cycling, but that seems like a poor reason to not use a bike.