Back in the dark ages, almost 40 years ago, when I started practicing law. We used to quibble over the "original" document. What was meant by this was that the Rules of Evidence had a provision requiring that the "best evidence" be presented in court. Hence, we had to make sure we proved our cases with original deeds, original promissory notes, original signed contract and etc. and etc. We would present the original in Court and have a witness testify as to the validity of the documents and the signatures. We would then move the document into evidence and then make a motion to substitute a copy of the original for the original for court purposes.
Flash forward to the present time. We hardly ever see the original documents, much less present them in court. If there is some issue about the copies or the validity of the original, those questions are all resolved long before a case goes to trial. Either the attorneys will agree as to the admissibility of the documents or not but in either event this is decided long before trial. Attorney's may object to a document at the time of trial but primarily the objections are "for the record." That is, to make sure the issue of the validity of the document is preserved should there be an appeal.
In short, original documents are almost never an issue in any court hearing. Copies are routinely used for almost all documents. Some vestiges of original document use still persist in stock certificates and wills but in day-to-day commercial litigation no one ever bothers to question copies as opposed to producing the originals.
Now, what about genealogy? Are we still interested in original documents or are copies sufficient? Think about digitized online documents. When we use online databases, we are quite a ways away from originals. But I think we actually use original documents more frequently than I did in the practice of law. Psychologically, we place a higher value on an "original" than on any copy. The issue goes back to the same considerations we have with book preservation. Are documents intrinsically valuable in and of themselves? Or are documents merely packaged information that can be reproduced in any format?
I think original documents whether they be certificates, letters, cards, or whatever are valuable artifacts in and of themselves. I think it is important that they be preserved as much as is possible. But I also recognize that all documents of whatever kind are really nothing more than information and that copies whether digitized or photocopied are equally as useful as the original, assuming that the copy shows sufficient detail so that the none of the information of the original is lost. As far as proof of the content of a document, I follow the legal position that a copy is as good as the original unless there is some issue about the validity of the copy.