I'd think that if someone sees you riding down the street with a big ol' helmet cam on your noggin they'll think twice about throwing stuff at you because you MIGHT have it on and recording, and you MIGHT catch them in the act, and you MIGHT be able to DO something about it after they've sped off.
So it could have a deterrent factor.
That being said, I ride pretty safely, and don't have any problems with that stuff... I think it'd be cool to have one, but they're still pretty expensive, and most of what I'd be videotaping would be me breaking various and sundry vehicle laws and filtering. Oh well, that link filtering video did look pretty neat (I like Cake) and it's probably what I would have done. The video is so grainy that you couldn't have gotten a plate number if that's what you had the camera for, but its still cool.