This question came up in the librarian section of the Comics Studies Society.
I argued in favor of keeping them as the art is frequently in an unfinished state, and can be thumbnails, pencils, or uncolored.
Here is an example of a proof, or Advance Reader Copy, of Trashed by Derf with an added editor's note and marketing information that may not be available anywhere else. It's from my own collection that I happen to run across recently, so I decided to take a few pictures to bolster my argument, and then thought I'd share them here as well.
I'd call this part of the Secret History of Comics.
My apologies for the so-so photos.
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