I belive the reason is efficency. If you're looking at the node directly, the entire pass devoted to processing <readmore>s is skipped. It seems silly to run the pass just so we can change it into a comment, when it already isn't doing any harm. Yeah, it makes it a little harder to validate, but that's a rather minor concern. (It isn't actualy that much of an efficency issue, either, but I don't see the benifit in writing the code to do it.)
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