Man- if I am going to go 60mph on my bicycle, the helmet has got to go...it's too hard to get my head between my legs to kiss my ass goodbye with it on!
OK- I'm chicken...43mph top speed so far.
As for the motorcycle guy- the reason for the half helmet is simple. The guys that wear them want to see and hear. I rode for ten years without one (and now with one- helmet laws) And I have to ask, how many times have you gone down? Now, how many have you hit your head?
The only person I ever knew who hit his head in a motorcycle accident...even in a full face helmet...well, I threw his ashes off a cliff into the ocean a week later (and his Ducati into a dumpster).
Not that I don't wear one- wife says no two wheeled activities without it and she's the boss round here.