I am coming to the realization that a helmet won't make you "safer" on a bike. Adequate and clean bike lanes, a well-maintained machine, courteous drivers (courteous cyclists), driver and cyclist education....those things can make you "safer." A helmet merely protects your noggin from when an unsafe situation comes home to roost.
I agree with Veronica. There are plenty of folks doing what we would see as jaw-droppingly stupid. I've been criticized far too many times trying to be everybody's mother to bother wasting my breath.
I, too, have seen more than my share of cyclist carrying their helmets - on their handlebars, clipped to a backpack, etc. Often, they are teenagers, and I suspect their folks made them get one, and couldn't leave the house without one, but they're just too "cool" to actuallly wear it. Sometimes, I do remind them "You know...they work better when they're actually ON your head!" I've even stopped and adjusted folks' poorly placed helmets as they sat on their head!
Maybe I need to work on that "I'm not everyone's mother" bit a little more!