1) This overview of helmet use relies a lot on the 2011 Elvik study -- unsure where Helmet Thread ended up regarding the legitimacy of this study.
2) The Elvik study indicates that while helmet use may reduce head/brain injury, it is only by a factor of 1.72 -- people who wear helmets still have every chance of sustaining the same kind of head/brain injury they wear helmets to prevent.
3) SWOV study also cites that majority of cycling injury, specifically head/brain injury is sustained by youth, not adult riders.
4) SWOV study also cites other studies indicating that mandatory helmet use decreases ridership and that those who wear helmets tend to ride less safe.

Science is fine, but where bicycle helmets are concerned, supports various arguments from both sides, does not offer a definitive answer.