I was recently rear-ended and my car was totaled, due to the fact that the idiot in front of me slammed on her brakes and the guy behind me couldn't stop in time. He and I both agreed that it was the fault of the idiot in front of me, and he was very glad his dashboard cam had caught the whole thing on video.
He told me he always drove with front and rear facing dash cams, due to the fact that somebody had attempted insurance fraud with him in the past.
Anyway, the insurance companies took care of me, but I'm sure he still got screwed, because he was the one that did the rear-ending.
Regarding this question of who legally owns the data in those dash cams, that's a very compelling issue. I'm more worried about the little black boxes which all modern cars now have, and which record data in the seconds leading up to an accident, data which can be used against the driver.