I don't wear a helmet on my uni commute (all roads) but I wear one nearly all of the time otherwise; don't wear one to uni because its a hassle carrying a helmet about all day and don't rate my chances, even with a helmet, if I get hit by a car….
I didn't used to wear helmets, but then I bought a "decent" helmet and just got into the habit of doing it. I've fallen off a fair few times, never hit my helmet. Dented one when I hit a branch but it I wasn't wearing a hat I think I'd have missed the branch with my head anyway.
Its just a matter of weighing up the risk. Yes there is always a chance of serious impact (even at slow speeds) but its not as likely as goign very fast down a hill or whatever.