The thing is, we're working with a general case. I'm looking at it from the point of view that it makes sense to start from scratch in order to learn or practice something new. Presumably the work re-submitted without modification wont score high marks since it will be somewhat off, but it will do as a lazy way out.
If the point of the assignment is to practice writing a paper (such as in a research methods or creative writing class), then I agree. In that case, the end result is not the focus.
However, if the professor is more interested in the paper than the process, then why do the work again? Having a previously written paper that has already been marked over by a professor is a great tool for improvement. Since the paper will most likely be for a different class, the paper will need to be modified anyway to fit the requirements of that particular assignment.