It's less customizable, for one (which, I admit, really doesn't matter to most people), and, in regard to just using valid HTML, a style element in the body is the lesser evil compared to a duplicate body element. But yeah; the biggest problem is that sometimes, it just won't work.
It looks like the rich text editor doesn't have a "hyperlink" or "edit source" button for whatever reason, so the only solution is to disable it, if only temporarily to enter <a href=URL>hyperlink text</a>.