Mark it depends on the country
In Australia, where it is now compulsory to wear a helmet, believe me it saves lives (as proven by the stats, i cant be bothered finding them now, but there available online). The reason is that in Aus, there are no bike paths and because our cities tend to be spread out with less traffic it means if you get hit by a car its guaranteed to be doing the speed limit and so will cause maximum damage.
In Germany, where i now live, I've never felt the need to wear a helmet. In Bremen (and most other cities) dedicated cycle paths run along the sidewalks (usually away from the roads) and so the only points where your likely to have a crash is at intersections, when someone is backing out of/or into their driveway or with pedestrians. None of which occurs at high speed.
So your right in some cases it works in some it doesnt... But even in small crashes I've never come across a case where wearing a helmet has been detrimental...