Surprisingly, there are very few studies on the actual effectiveness of bicycle helmets. Lots of times, articles will just say "X% of people that are injured aren't wearing a helmet". Which of course is next to worthless.
Helmet laws make the general public less likely to cycle, and as many would then drive instead, thus have an overall negative effect on public health.
I think my favorite stupid argument though is that it increases the size of the rider's head, which increases the likelihood of ground collision in an accident. As though it's so easy to stop yourself from falling as long as there aren't two inches of material strapped on your noggin.
The basic fact is that if you do have a head injury on a bike, a helmet is likely to reduce the severity of it. Everything else is an unquantifiable argument about human behavior.
The main negative effect of helmet laws that I've seen cited is that they reduce the popularity of cycling as a transportation mode, and that can have extremely negative effects on public health (due to increased driving, reduced physical activity, and indirectly as a general damper on pro-cycling sentiment and infrastructure changes).
[I think Australia's helmet law is one of the main examples people talk about.]
In the end, it's far more important (in terms of safety, as well as for other social goals that cycling promotes) to get people on bikes than it is to micromanage the way they do it.
It's worth nothing that helmet usage is extremely low in pretty much all the great cycling nations (Netherlands, Denmark, Japan, etc), all of which have cycling mode-share orders of magnitude greater than any place in the U.S.
[In Japan it is becoming increasingly common to see very young kids wearing bicycle helmets, especially when they're seated on their parent's bike, but they seem to stop wearing them sometime in their grade-school years.]