A couple reasons. The Elmo is engineered to have any portion of the film that makes full contact to the film surface to be undercut, or rubber. The urethane grips, but does not slip on the film as it is being projected. This does not cause abrasions on the film. Plastic slips and doesn't grip. There are no projectors made now, or in the past, that have flat hard rollers that contact the full surface of a film. The flanged roller is engineered to keep the film in a straight line over the sound pick up. Since there are 3 kinds of flanged roller, the plastic substitute without rubber causes the sound to be misaligned over the sound pick up in many cases. If hard plastic rollers were the best way to make the rollers, Elmo would not have gone through the cost and extra expense to fit the CL with rubber rollers. Basically plastic rollers are a poor substitute for the proper part.
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