I don't think that serves to mitigate loss of evidence.
There are procedures in place to allow video evidence to be retained for use against a suspect. When that is done, it actually makes the camera video evidentiary for every crime it records.
To put it simply: the police don't get to decide the recording is evidence for this crime and not for that crime; it becomes evidence for every crime. That's because the Constitution does not allow the police/prosecutor to pick and choose which evidence the defense can see or bring to court.
Such a procedure as you describe would be incompetent. Because you can bet the officer would drop off the recording of a guy-who-hit-an-officer (for example). If the officer can and will do that, but shrugs off the DUI-arrest recording, well the officer becomes a decider of which evidence the defense can see.
"If I have a pile of NSL's and I count them: 1...2...3...4...5...6... and then I publish that there are '6' NSL's, well then...
"Each of those NSL's is part of that count. And if you just take a properly charged magic wand and wave it over that '6' then you can scry each of the individual NSL's that made up that count.
"As you can clearly see, that would expose the contents of each of the 6 NSL's. So we must take care to keep that '6' confidential, just as we keep the NSL's confidential. That '6' is six times as secret as each NSL!
Suppose, for discussion, that the defense is short of time, money, or brains: in which case the agent-of-government may not even have come up.
So from the prosecution perspective: better not to mention it and hope it never comes up. This is how citizen rights are "innocently" violated wholesale, and it happens all the time.
Worse, it only came up because the Best Buy agent was paid. Suppose a different scenario: the Best Buy agent was cooperating to avoid prosecution for something they did. Well, that makes it all better, doesn't it, to have them directed by the FBI and no money changing hands--and also much harder for the defense to prove.
The problem is, first, the direction of a confidential agent by the FBI (whether paid, quid pro quo, or volunteer) and, second, the FBI's (repeatedly demonstrated) willingness to conceal the fact that the agent was under FBI direction.
The bottom line was that the FBI hoped it would remain unnoticed that such a relationship existed. As they see it, just bad luck that this time it got found out. Better luck next case.