IMHO... flipping someone off is vulgar, threatening, and at least one step (maybe the only step needed) towards a violent interaction. However... it's unlikely the police (around here anyway) would react to a video of someone flipping you the bird. So... what's the point of recording it?

With that said... I have a handlebar/helmet mount camera. I haven't as-of-yet recorded any of the interesting cycling footage I thought I might when I bought the camera. I think mine was a better idea... than a useful tool.