Body camera footage has been used to convict police of crimes where otherwise they would walk free. Until the system we live in drastically changes for the better, it's wiser to be aware of every tool at your disposal should you, or someone you know, be put into a situation involving a legal battle with the police.
A cop turning off their body camera is evidence in proving they are untrustworthy and therefore more likely to be guilty. We know this, but judges and juries need to be convinced as well.
Personally, a cop turning off their camera should mean instant termination, fines, and jail time, but as we've seen, they do not fear retalliation for their actions.