Yes, it will likely happen. However, that is, in my opinion, insufficient disincentive, for the following reasons:
1.) If it "accidentally breaks" 50% of the time, it still means that half the time it's working, which is higher than the 0% we have now.
2.) secondary units could be kept in the glove box; most juries would have a very difficult time believing that both cameras failed, or that a known-dangerous situation wouldn't warrant having both cameras on anyway, or that both police officers involved both had faulty cameras, or if only one went in that he/she was not following protocol....basically, the lack of evidence when there damn well should be would lend more credence to the victim than the police officer, leaving it in the officer's best interest to keep it working (or report it malfunctioning sooner than later).
3.) It would help curb selective enforcement; officers would be more likely to more fully follow protocol.
4.) random footage audits, like random drug tests, would assist in internal investigations; officers whose cameras are 'accidentally broken' during an audit would be much easier to penalize, again, keeping it in the officer's best interest to avoid having a malfunctioning camera.
5.) "I have nothing to hide" is a reason frequently given for giving up one's privacy when prompted to do so. If it's true, then "I have nothing to hide" should most certainly hold accurate for people on the public payroll.
6.) A highly trivial reason, compared to the major ones: checking cameras and footage in and out is a good way to add a few dozen jobs to the local precincts.
It will happen, of course...but if it even partially helps the situation at hand of "he said she said" where either no one trusts the cop (in cases where the officer was either genuinely right or ultimately wrong, but in a split-second decision situation), or victims of police brutality are further victimized by the 'ol boys club', then I'd say it's a hell of a much better use of both my tax dollars and Seagate hard drives than the use of either by the NSA.