Long piece from the August issue of Climbing about why so few climbers wear helmets:
When it comes to helmets and climbers, inconsistency is everywhere. Most ice climbers and mountaineers wear helmets, as do many traditional rock climbers. But far fewer rock climbers don lids for short climbs, especially sport routes.
See also this piece on The Stone Mind about the dangers of sport climbing without a helmet and this piece on Mountains & Water about how wearing a helmet while bouldering might make more sense than you’d think.