I mean, you would expect some improvement, but (as a Dutch citizen), i expect it to prevent harm in the same way wearing helmet during sex would prevent harm.
In general: if you taking those kind of risks (as a car, as a bike, etc.) that people on a bike need a helmet, you don't belong in traffic here. The effect on mentality may be as important, if not more important.
Also, interesting to realize, if car and a bike have an accident, by law, it is automatically the fault of the car, no matter what. Even if the person on the bike is drunk, driving in the wrong direction, etc.
So, although bikers are betting their life going out, cars are betting their wealth, money and freedom.
It turns out, instead of focusing on 'who is at fault' and 'who is right', it is much safer to focus on 'lets make sure we don't hit each other' and turn accidents into a mutually-assured-destruction kind of deal. This way, every actor in traffic has a natural tendency to compensates for mistakes of the other actors. And this attitude is the one that prevents most accidents.