I find it hard to believe a copy of evidence is evidence unless one were to introduce it in court as such. I'm not quite interested enough to research the question, but think of the practical issues of introduction.
Say the DA gets a copy of the tape from the defendant's attorney and tries to introduce it.
Defense Attorney: "Objection, lack of foundation."
So, we then get to have a chain of custody foundation to build which could include the defense attorney and the defendant. After they get the technician to testify that he made a true and correct copy and the secretary placed it in the mail properly or delivered it to the DA...well you get the idea. Then the real problem(s) of the defense attorney can be gotten around, but the defendant? "Ahh...I decline to answer on the grounds it may tend to incriminate me."
Now, having a defendant testify that when his attorney gave him the copy of the tape to review he kept it in a secure place and didn't tamper with it may not be testifying against oneself, but I bet we'd go through some appeals before getting there.
For a drunk driving?
Of course its evidence. It is a video of of the officer's investigation of the alleged crime immediately after it happened and immediately before arrest. Its GREAT evidence. It doesn't matter who is in possession of it, how they got it, or whether there are additional copies. It truly doesn't matter whether it is admissible in court or not. Evidence is anything that could potentially be probative, and the absolute first step in determining admissibility is gaining possession of the evidence so that it can be inspected...which is why the State has a right to it if the defense has the only copy left. They have a right to it because it is evidence, because they need it to determine its admissibility, and because they need it to evaluate all of their other evidence.
Something isn't turned into evidence by being admitted in trial. If that were the case no one could ever be guilty of tampering unless they ripped up the exhibit after it was introduced in court. The whole point of a tampering charge is that the person altered or destroyed the evidence specifically so that it couldn't be admit in trial.
I think maybe you mean that the video is not the best evidence, which is an evidentiary issue but if the original and all other copies are missing than the defense's copy IS the best evidence.
As far as proving up the predicate and admitting it in trial, that is not at all how it works. You never prove up the predicate for a copy, you prove up the predicate for the original, even if it doesn't exist anymore, and then you establish that the copy is true and accurate to the original. You almost never admit the original tape in a DWI trial, you admit the State's copy. The officer testifies that the original was filmed on a device capable of making accurate recordings and that the operator was competent. Then he testifies that he has viewed the copy prior to testifying and that it is a true and accurate copy of the original. He would do the exact same with the defense's copy if it was the only one available. Copies are frequently admitted over originals and this is true regardless of what generation copy it is. Think about any certified document like a judgment. The original copy is in a warehouse somewhere. They scanned that into their computer and they print you off a copy. You now have a 3rd generation copy, but they put the seal on it and that verifies that it is a true and accurate copy and it is now self authenticating in trial. Many clerk's offices have copies which are a copy of a copy times 100. It doesn't matter because they are all certified copies of the original, including the copy they give you which is the 100th generation copy.
As a prosecutor I admit the defense's copy of evidence in trial many times. Sometimes I had to fight to get it because our original was lost or damaged, other times I did it just to make it easier, (ex: did agreed redactions and made them a copy to review and then admit that copy so that they knew the redactions were correct).
The bottom line is that if you look at something and say to yourself, "this might influence a jury's decision even sightly," than that is evidence.
