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Keep in mind I haven't had time to test this extensively. With that assumption, using an adjective will put more emphasis on it and the results will suffer. Comments?
When you search for blue widgets, this allows them to give more weight to a page with "blue widgets" than a page with "blue" and "widgets" in different parts of the document. Also, it lets them give more weight to a page with "blue widgets" than a page with "widgets blue".
Having a default proximity weighting was one of the things that made Google's results different from (and I would say better than) other engines.