It's kind of like the reasoning behind a motorcycle helmet--in the reare event you do crash, and in the rarer event a head-strike is involved, a helmet may actually help with injury mitigation.

For the speeds involved, despite their flimsy appearance compared to a full-face moto-helmet, a bicycle helmet will probably help very much in cases where there would be minor injury, may help in cases of moderate injury, and do provide some protection even in cases of severe head injury.

But just like with a motorcycle helmet, there will always be freak accidents where they will be of little or no help at all. When I started bike commuting on a regular basis, I was coming from an All The Gear All The Time motorcycle background, and very naturally decided to wear a cycling helmet. In 15 years of wearing one, I'd say it may have helped me once avoid or mitigate some kind of head injury. Beyond that, I like the visor and as a good place to hang a flashing rear light.