I've seen that stated many times. As far as I can tell, it traces back to a news article that did fun things like cherry picking individual observations from a study to support a claim even though the study's overall findings were contradictory.
My own sense is that the science seems pretty clear that, provided you are in a wreck, wearing a helmet greatly reduces your risk of serious injury and death. Helmets aren't perfect, and they may primarily be designed for lower-engery impacts, but even insufficient energy dissipation is better than zero energy dissipation.
There's a secondary claim that not wearing a helmet results in safer behavior than wearing one, and that outweighs any potential benefit. I'm not aware of any quality research to back that claim, and you'd never be able to get a study to pin the question down past an institutional review board, so I'm inclined to say that cuts very close to what we called a fundamentally unanswerable question back in my class on experiment design. (Well, actually we used the acronym most the time. ^_^ ) My personal sense is to think that expecting your garden variety distracted, chronically fatigued, or nonzero BAC driver to notice whether you're wearing a helmet in the first place, let alone modify their behavior accordingly, is giving them more credit than they've earned.