That's the thing though, burn marks, scratches and colour inconsistencies are just as much due to the technological limitations of the time as matte lines or rubber masks. The colourtiming process was much more difficult then than it is today with digital technology and that's why it couldn't be as perfect as today. Same goes for handling the film. Today, most of the editing work is done on a digital scan of the film, back then it was all done by hand in a lab and so accidents sometimes happened and little scratches and burnmarks appeared on the original negative and became a part of seeing the film. Some could argue that a matte line around a model takes you out of the film or seeing a wolf-man in the cantina takes you out of the film, and they may be right but that doesn't mean it should be removed.

@Jan: Well, this is something that happens every time there is a wipe, the noticeability of it depends on how different the colourtiming is from shot to shot.