I cycle with a helmet cam, and I typically inform the driver that they are being recorded. While I haven't done a scientific study, I believe this causes some drivers to restrain themselves.

I've only recently installed cams (front and back) on my bike. I hadn't felt the need, but a couple of weeks ago a friend of mine was pelted with eggs while he was riding.

I decided that being able to gather evidence and, if lucky, license plate numbers, is not the worst idea.

One only hurts for a while.