Coming from a MTB background there is basically zero doubt that helmets prevent many head injuries, i have seen some nasty falls in rock gardens and into trees.

Will it safe your life if you're hit by a car? Meh... from what i've heard most cyclists who are hit hard by automobiles and were wearing helmets die from massive internal organ trauma. Those are the types of accidents that will probably get you anyways, no matter how one slices it. Thankfully they are in the minority. As for the majority of the rest of the accidents on the road like getting doored, catching your tire in a streetcar track or knocked off by a pedestrian, helmets do make a difference.

I'll keep wearing it, i don't feel people should be forced to if they're convinced the helmet is of no use... it just makes for a lot of bitter resentment.