Here's one in favor of the headgear: in the event of a collision, it will take the crack so your head doesn't have to.
Our sales associate Jakub was riding into work this morning down 1st St. when he rode over a small scrap of wood. It seemed unassuming enough, but the scrap kicked up between his front spokes, locked up against the fork, and stopped his bike in its place, sending Jakub over the bars.
The front of his Giro helmet took the brunt of the collision with the pavement, and Jakub credits the helmet as possibly saving his life. Even with road rash on the nose, chin, arms and legs, and a bent fork and spoke, Jakub continued his commute into work (all teeth intact too!). He'll need a new helmet and fork, but feels lucky knowing it could have been much worse.
You may do the same route hundreds of times on your bike without an accident, but wearing a helmet keeps you safe for that one unfortunate moment.