It's not about recording speed or racing.. It's incase someone hits me or causes an accident and blames me. Nearly every incident I've read about, the driver admits fault on the scene however proceeds to blame the innocent driver 2 weeks down the line.
I've been in 2 accidents (in the passenger seat of my Dad's car) where that exact thing has happened. One being rear-ended whilst stationary, then being accused of reversing back into him. Second one where a woman in a Peugeot stood on her accelerator rather than brake on a country road, slamming into the side of our stationary car, only to end up claiming we rammed her and gave her whiplash...
At the end of the day, if someone does do that, the camera would pay for itself several times over and prevent a whole lot of aggravation.