I almost always wear a helmet and I recommend other people to wear one if they like to go fast, but BMJ is "dead" on. Danish medical researches have been researching the subject the last couple of years; commuting on bicycle, even very short distances at low speed, dramatically improves peoples health.
And a 20 minute a day cycling habit where one cycle so fast that the heart gets a little workout adds 3-5 years to peoples lives in average and dramatically reduce the chance of getting one of many chronic modern "lifestyle" diseases like diabetes. These studies have been rather thourough and cleaned for social and economic bias etc.

So if the choice is between cycling and not wearing a helmet, one should choose cycling every time.

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Regards