With head injuries due to impact, you do not need to fracture the skull to injure the brain, and, yes, direction of impact matters. Think about your brain as floating in a thin layer of fluid contained in a sac (the meninges) with spots attached to the skull. There are also little veins running from the surface of the meninges through tiny holes in the skull (emissary veins). If you are moving forward, and the skull is abruptly stopped (impact), the brain sloshes forward. There is a tent of meninges in front of the brainstem that it smacks into in that type of collision, and can be very abruptly fatal since that part of the brain controls vital functions. Less severe injuries can rip those little veins and cause bleeding and compression of the brain. But, just the sloshing about (think about how a big bowl of jello wiggles) can cause more diffuse brain injury too (much of the severity of a gunshot wound to the head is due to the ripple-effect of the wave generated as the bullet passes through rather than the hole it makes, per se). If you get injury, you get inflammation and swelling. The brain swelling and squishing through holes it's not supposed to go through results in a lot of the slower reactions to brain injury. This too can be fatal.
As for skull fractures, the skull is not uniform thickness. A lateral impact to the temple is more likely to result in a fracture than a frontal impact. There's a major artery running along the temporal bone, and much the way glass breaks most easily where you score it, the skull breaks most easily along the groove where this artery runs, and then cuts or shears the artery. In other places, a fracture alone is not so serious as long as it isn't compressed pushing the bone into the brain.
Hitting the very back of your head doesn't hit the brainstem...that's tucked further under and usually well protected...except from that tent in front of it that is supposed to protect the brain except in sudden collisions. The part of the brain that perceives vision is all the way in the back, so hitting the back of your head might blind you, or at least make you see stars (a visual hallucination), but won't kill you unless a blood vessel ruptures.
So, probably the most important part of a helmet isn't the hard shell that prevents fractures and general scraping along asphalt, but more likely, the padding inside that cushions the impact. On the other hand, in talking with a first responder on the issue, her impression (anecdotal, but from many incidents) is that if you hit head-first into pavement or another vehicle, it really won't matter. They just call the helmet a brain bucket and usually the victim is DOA. Where it's likely more useful is if another part of the body strikes first and absorbs a lot of the impact, such as a shoulder, and the head hits pavement secondarily, scraping along, or you tumble and roll before your head strikes anything.
According to neurologists I've discussed this with, the most important aspect of a helmet is reducing the severity of the injury you will survive. Keep in mind, they only see patients who survived the accident and made it to them. So taken together, I'd interpret it that a helmet might not help you survive a motorcycle accident, but if you do survive it, you're less likely to be a vegetable and to make a more complete recovery.
