If you fall off the bike going just 10 mph, you will be in much better shape afterwards with armor on your knees, shins and elbows. I have heard of people falling over while stopped at a stoplight and dying from a head injury because they had no helmet on.
I rode dirt bikes for years, and picked gravel out of my knees, elbows and in the palms of my hands, along with almost getting knocked out, breaking a toe, breaking 2 ribs and breaking 2 leg bones, before I decided that helmets, gloves and other armor were a good idea. Most of my dirt bike crashes were at less than 30 mph, going up a hill or in a tight corner. The dirt bike armor tends to be cheaper and not as hot in the summer. Pavement scrapes your skin off faster than dirt.
I also wear one of these below for my street bike- long rides are less tiring, and it reduces bruises from a crash:
http://www.locox.com/Motocross-ATV/Motocross-Kidney-Protection
