Just an anecdote with a sample size of 1, but I hit my head while cycling this summer for maybe the first time -- I've come off the bike before, but never landed directly on my head. This time, I managed to spin out in some dry leaves, fall, and stop my fall by hitting a frickin' railing with my head. Even though this was at very slow speed (I was riding up a spiral bike / pedestrian overpass to cross a major street) I would definitely have had a concussion - at minimum - without the helmet. I will continue to wear a helmet while cycling and boarding (and skating, for that matter).