I have been thinking about getting a helmet cam to mount on my rear rack. It would be nice to stop at a police station and show them the video of a driver who purposely buzzes me (I wonder if seeing someone do it on video would get them to act, I know just reporting it doesn't get any action), and I would find it comforting to know that my next of kin would have some nice evidence to use when suing someone for wrongful death after they killed me by running me over. Also no need to memorize a license plate number.
Of course it would also be fun to make some videos of rides too.