Headcams can be useful for biking too. Here's a video news report
of two cyclists who were hit in the Berkeley hills of BAOC country by an SUV, apparently on purpose. One of the riders had a handlebar-mounted cam which recorded the hit-and-run incident and the driver's license plate...and allowed his subsequent arrest. Unfortunately past history indicates few drivers receive anything more than a wrist-slap in such accidents. Perhaps the graphic nature of the rider's video, documenting the flagrant crime, will spur the court to invoke greater punishment and jail time.
However judging from some of the comments appended below the article, a headcam might be appropriate for ANY bike ride in the Bay Area.