I have been debating this topic myself. Many people wear bike helmets, but more do not. The most common reason for not wearing one is usually "It looks bad and ruins my hair". When you think about it, that statement sounds vain. Sure not every person who doesn't wear a helmet gets hurt on a bicycle. Sure you could be the exception. But are you really willing to take that chance?
So for those who want to be safe and fashionable, here are some ideas. Put your hair in a low ponytail or braid. This way the helmet is secure on your head, but you still have a cool look. Or just wear it down. Go ahead and experiment with a helmet and your hair. When you take off your helmet, and you feel as if your hair is a mess, use your fingers to pull your hair back into a slightly high ponytail. Don't worry about the bumps that come by not using a hair brush. Actually, the messy hair style can give you a chic "I did this in three seconds, but still look good" kind of look. If you don't trust me, just look in the mirror and ask the people around you. I bet you most will say your hair looks good or fine.
So wear a helmet and use the mentioned techniques. Saftey doesn't have to mean ugly.