That's a difficult question to answer... my belief is that the increase in head injuries from increased rotational forces is about the same as any reduction in head injuries due to the impacts that helmets can help with.

My real concern in regard to helmets has more to do with the image that is created of cycling being 'dangerous' and thus helmets are a 'critical' piece of safety gear, and the concentration on helmet usage to the exclusion of other bicycle safety measures that bring about real increases in bicycle safety. Touting the supposedly huge dangers of riding without a helmet also tends to make people more afraid to cycle, combine this with possible mandatory helmet laws and you typically have a very sizable decrease in bicycle usage.

Reduced bicycle usage makes it more uncommon for motorists to interact in meaningful ways with bicyclists, thus reducing safety because motorists are no longer watching for people on bikes.

Combine this with a reluctance on the part of law officers to enforce the laws as they exist and you end up with a downright hostile environment for cyclists.