This way you could give an educated guess of what are the most important keywords using a modern approach that is also used by search engines.
It also is more likely to catch the intuitively meaningful words of a text: We all know, that some words are more common than others. While having a hard cut between words that are too common to be included and all the other words is certainly better than nothing, having a smart algorithm decide exactly how common a word is almost certainly even better.
Note: As a side benefit, you don't have to exclude common words (and, I, you, etc.) by hand anymore, because they get weighted down by automatically.
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