Much like WZ touched upon, even if it weren’t for the fact that it can save your life, with the bugs, pebbles, etc, that’s out there, why would you consider riding without a helmet?
I remember last summer I was pretty excited, I had a perfect afternoon off, I was on Vermont Rt. 17, a legendary motorcycle road, for the first time on my bike. I was about to hit the first serious twist when this giant green thing-either the biggest bug, or the smallest bird, in the state-exploded on impact with my face shield. I had to pull over and find some tissues to wipe it down; truly gross, but a minor inconvenience. If I hadn’t had a helmet with a face shield it would have been traumatic, and probably pretty painful.
As for protecting my noggin, I’ve been wearing a helmet to ski(and snow is usually a bit softer than pavement) since the mid 90’s, when it looked rather odd to see an adult with anything other than a wool hat. My bike may be naked, but my head will never be.