I think that some archivists are happy even for "dirty" copies because it keeps people's hands off of the originals. And I mean that literally, because some of this stuff is so old and delicate that it will easily break apart in your hands. In the UK National Archives (at least in my room) they won't let you look at an original document if there's microfilm of it, no matter how bad. If it was unreadable you had to get an archivist to agree before they'd sign off on you requesting the original. I had one archivist tell me no and it led to an especially difficult day.