Look up the important but (oddly) rarely used “best evidence rule” as a piece of legal doctrine.
It says that an original document should be used versus a copy unless the original is unavailable.
Now IANAL but I have worked as a paralegal and my brother is an atty and judge so we discuss these types of things all the time. But to me, this is not just the FBI using [an innocent] Xerox copy of a document: It is deliberately ratifying the use of a compromised document.
Kind of like, you know, 0bama’s birth cert. You can have whatever opinion you wish as to 0bama’s birthpace, but the idea that the document submitted as evidence of same has ten or more edits on it means that you know, from the outset, you are accepting garbage as your evidence and the other side knows it is garbage and is just waiting for and yet demanding you accept it. You know you’re being lied to at some level.
It goes to the issue of authentication ... the more difficult it is to authenticate, the more open to question its validity and/or its content. Also, if it can be determined that it is a valid copy of the original document, it is still possible the contents are fabricated, or added long after the event, etc etc.