The only time I "used" my helmet so far was on an MUP, not in the street.

I'll skip all of the details, but it wasn't severe, but would have been worse without it.

What makes me wonder is when I see people riding with their helmets strapped to their racks, or hanging on their messenger bags instead of on their heads while they are riding in traffic. I saw that at least twice in Seattle in the last week. I just thought "Why do they even have a helmet?"