I once fell at a bowling alley when I stepped onto a section of freshly waxed lane just inches past the "diamonds". My feet flew forwards from under me, and I fell on my back, striking my head nearly hard enough to lose consciousness, and plenty hard enough to give myself a headache. Given that I was only moving at 3-5 mph, I wonder how I'd have done if I had fallen from a faster moving bicycle?

One suggestive answer comes from an experience in which, in 1975, without a helmet, I jumped an old motorscooter on a bicycle ramp some kids had built on the sidewalk down the street from my house. The scooter lept into the air in a very enthusiastic manner, but it's rear wheel rose much further than the front. I went over the bars and landed almost face first on the sidewalk. I WAS unconscious for a while this time. Bruised badly, with a "Frankenstein's monster" scar on my forehead, but nothing broken, and a VERY severe headache which lasted a couple of days.

I purchased my first bicycle helmet around 1995, and found a couple of things:
I absolutely don't even know it is there. Get a decent fit and it is not distracting in the least.
On a sunny day, it promotes cooling by keeping most of the top of my head shaded.
This is especially true for people who select a helmet color lighter than their hair, and for people with thinning hair.

I have never heard of anyone dying because of wearing a bicycle (or motorcycle) helmet, but plenty who have died because they didn't.