If I was frequently riding at speeds over 30mph I would want to be wearing full leathers and a motorcycle helmet. Crashing at 40mph is pretty much guaranteed to end in death according to the statistics.
I have and didn't, it was wet and I slid a lot, hurt like hell. Didn't bang my head tho.
It's hilly here so frequently see over 30mph, biggest danger with that is still being pranged by other road users who don't think you're going that quickly so I slowed my commute down as too many T junctions to pass. I've only broken one helmet, part of the damage was a pierced hole from a rock.