Over my lifetime riding bikes I have witnessed two major crashes where both non helmeted victims survived with life changing traumatic brain injuries.

One fell off a low bridge and the other was riding causally along and somehow ended up going over the bars and hitting her head on the curb.

Seeing just one of these was all the reason I needed to never ride without a helmet.

I wear a couple types depending on what sort of riding I'm doing (mountain, road, racing) they are MIPS equipped when ever available.