No safety device is guaranteed to save your life under all conditions. However ... a helmet, even a bike helmet, CAN make the accident more survivable.
I read an article a few years ago, where a mountain biker went over the handle bars, head first into a rock, about 10 feet below. The helmet split open in the middle, he had a gash underneath where the helmet split, a concussion, and a cracked vertebra. But he lived to ride again, with no permanent brain trauma. Had he hit the top of his head on the rock without the helmet, it would have been his skull that split open.
Now, if he had hit the rock harder, he might not have survived. But, a helmet gives you a better chance of surviving a given accident. IT absorbs some of the force that would otherwise go towards damaging your noggin.
I look at it this way - Wearing a helmet is one way that you can stack the deck in your favor, in the event of a potential head trauma accident. It's NOT a magic shield, and it may not work as you hope. But, it improves the odds of you still being the same person, if that day comes.