That would be doing violence to the study. When a rider gets strained through a chain link fence they should be eliminated from the study, similarly when a rider's abdomen gets crushed under a truck a helmet is rather irrelevant. The point is that there are sufficient "other" causes of mortality that mask the helmet vs. non-helmet signal unless one confines the study to head injuryassociated deaths only.
Thal, the problem is that they (doctors reporting) don't do this. If a guy crashes into a truck that pulled a left in front of him, and his ribcage is crushed and his skull broken, if he is not wearing a helmet, it is a head injury.
That's why that study you quoted talked about reducing head injuries. No argument there, but that isn't the only thing that kills. Head injuries are an easy thing for a doc to point to.
When it comes down to it, helmets will save those who would have died of head injury, but most wrecks are such that the rest of the body is smashed.