Its competely irrelevent to the issue of whether helmets are safe or not.
To be all serious for a moment, there are two completely different, but related questions. They are perpetually confused by people who really ought to know better.
1. Does wearing a helmet make the individual rider less exposed to risk? That is a factual question which is open to scientific and statistical analysis.
2. Should helmet-wearing be made compulsory? That is a political question whose answer may be influenced (but can never be determined) by the answer to question 1.
The answer to question 1 is "no-one knows - there is a mass of contradictory evidence, and the answer will probably be conditional on the specific individual rider".
The answer to question 2 is, at the moment, unequivocally "no" - both because the evidence from Australia and elsewhere is that the introduction of such a law results in a bad outcome (fewer cyclists) and because it is important not to unnecessarily restrict personal liberty. It is possible see circumstances in which the answer might become "yes".