From The DD News Blog: (click for full article)
This is from an article at MLive, "Analysis: Motorcycle helmet use falls, injury rate climbs since law change" by Jonathan Oosting, 7/19/12. It shows the evidence so far, that Michigan's repeal of its motorcycle helmet law has caused more deaths and incapacitating injuries (i.e. disabilities) for motorcycle riders not wearing helmets.
The good news is that at least for the first two months after the law was repealed, 80% of riders involved in a crash continued to wear helmets. For the third month, that figure dropped to 68%. According to the article, a spokeswoman for the insurance company State Farm of Michigan said the findings were consistent with a National Highway Traffic Safety Administration report.