I read in an issue of a mountain bike magazine (sorry I can't remember the issue or even the magazine), that helmets afford somewhat limited protection. That is not to say that you shouldn't wear a helmet.
The study showed that helmets can worsen head injury in crashes involving torque to the brain (i.e. rotational injury). Rotational injury is not like the relatively benign bleeding from simply a scraped scalp (though no head injury should be taken lightly!). Since the brain is not perfectly round but elongated, rotation can happen more easily at a pivot point. Torque injuries are the source of the most serious brain injuries.
Also the study seemed to suggest that helmets are most effective at protecting you in high speed crashes and crashes into sharp objects, which is particularly important in mountain biking. At medium to low speed crashes, the helmet's effectiveness was in doubt.
Other non-mountain bike specific studies seem to have shown the frequency of head injuries compared to other types of injuries is rare. The statistics can matter in deciding if helmets should be mandatory because if you also have comparable amount of spinal injuries then you could argue that it should be law to wear back braces like those DH mountain bikers wear, even on the road! As a consequence, it is understandable why some people think helmets are an arbitrary requirement.
But those studies also need to take into account in what environment these injuries take place. E.g. in Amsterdam and most European countries no one wears helmets but bicycles and cars move much slower and because of their infrastructure they do not have to meddle with cars as much. It's a different story in places like USA and Australia where there is little or no separation between bicycles and cars, and cars move very fast.
Anyway I always wear a helmet.