I wear my helmet every time I go cycling and have done for the last three years. I started wearing my helmet as soon as my wife became pregnant - she, rightfully so, insisted on it. If I was to have an accident now, my helmet might just help my kids still have a Daddy to help support them.
I've had a couple of accidents from drivers not seeing me (despite a florescent jacket and lights) and pulling out of a junction in front of me. Luckily these were all low speed as I could spot that the drivers were not looking so not much damage to myself or my bike. In both accidents, a helmet made no difference, but if it reduces the risk of major injury then it makes sense for me to wear one. Also, being married means I'm less concerned how messy my hear is from wearing a helmet than when I was single!