Are SO important.  I don't mean to sound old or anything - but one saved my life and another prevented a lot of injuries.   
I wouldn't say I was exceptionally  wreck-less on a bike, but I did have a LOT of fun.  I raced alleycats (where I was hit by a car and rolled off the hood.  Won first female but found a crack that ran all the way from the outside to the inside of the helmet after the race - saved my life), played a lot of bike polo, raced unofficial track races (responsible for the loss of another helmet - over the handle bars) and wove through traffic (ok, that was wreck-less and irresponsible.  7 traffic lights and got off with a citation and no injuries.  LUCK-EEY!)
My point comes to this.  Soren has two bikes.  A Strider and of course, a tricycle.  And he is getting brave.  I mean - really brave.  He's a bruiser and can take falls, but helmet wearing in this household is a non-negotiable.  My dome would be scrambled eggs without one and I still feel the pain of a knee injury from another cycling accident.  I know how dangerous they can be, even in the drive way. 
But cycling is so much fun and I want Soren to love it too, and he does!  I don't shy away from telling him just how dangerous it is to ride without a helmet.  It isn't always easy convincing, but we are getting there.  He tells me, 'Ride Bike!  Helmet peeease! - fall, OUCH! hurt, waaaaay bad!'.  Want to get something fun, check out these little nuttys! 
Soren has this one - gotta figure out a way to scratch off NY and replace with OH, or QC (for Queen City).