I always wear my helmet... by choice. Every time I strap it on, I anticipate that it will be a total waste... but I've had a couple mishaps in my lifetime where it at least saved me a lot of pain and rehabilitation, if not my life. (Power-slammin' your head into the pavement could mess things up - that styrofoam can soften the landing up considerably.)
Some well-meaning folks believe that helmet use should be mandatory. And in fact, there are some helmet controversies going on, north of the border up Canada way.
In Ontario, the chief coroner is recommending that helmet use be mandatory. He cites some interesting figures... only 27 percent of cyclists killed in the province between 2006 and 2010 were wearing helmets. (Story HERE.)
Meanwhile, 1500 miles or so west of there in British Columbia, helmets have been mandatory since 1996. Although nobody argues that helmets make cycling safer, critics argue that people are dissuaded from riding bicycles by the law. Chris Bruntlett has a bike-advocacy group, "The Church of Situp Cycling" (clever!). He says "We have seen the impact of the mandatory helmet law and it has killed off the idea of a short, spontaneous and slow bicycle trip... where you go out for a coffee or groceries, the idea you need (head) protection discourages people from riding and they will take the car or bus instead." (Story HERE.)
As much as I'd like to see all cyclists wear helmets, I can't support the notion of laws mandating it. Adults should be able to make that choice. And that choice is likely driven by the circumstance... a long road ride might merit a bucket, a quick trip to the corner market, maybe not. (I wear a helmet if I leave the driveway. But others should be able to choose.)
My advice, as always, would be, "If you use your brain, wear a helmet. If you don't use your brain... well, it probably doesn't matter so much."