Everyone likes accountability when they have control over it. The cops would have control over the tapes, right? So they get to choose which parts to show and which parts to "inconveniently lose."
One small problem with that theory... if they "inconveniently lose" a critical bit of video evidence at trial, the defense would savage them for it, and the jury is likely to let that fact color their decision in a way that is not advantageous to the prosecution.
All said, since most prosecutions end up plea-bargains this may be moot, but for those that go to trial...?