I'm all for police wearing cameras, provided that they cannot disable them. Unfortunately, it sounds like these can be disabled.
Don't think for a minute that the police intend to use the videos to clear suspects of wrongdoing, or to have documented evidence of police abuse that can be used in court.
It's the other way round: the police want video evidence to prove either that someone was rebellious during their arrest, or that the police didn't abuse them. People often try to blame the police in court to get off easy, and the police always has to prove they didn't use undue violence during the arrest. So videos are a good idea.
Unfortunately, it's a rather one-sided protection, as the police would never show videos in which they'd appear to have abused their powers. On the other hand, the lack of video itself would appear extremely suspicious in court I suppose, and would bolster a defendant's claim of police violence.