I may be misunderstanding some of the legalities of the SRD and its usage, but what if someone wanted to make a 3.5-compatible Prestige Class? The term "Prestige Class" is not part of the 5.1 Creative Commons release. One could argue that the term is generic enough to use safely, but if I was a small publisher, I would not want to risk a C&D letter from Wizards. There are other terms and phrases unique to 3.x play that were not carried forward into the current SRD. You could rely on the (as yet) untested-in-court notion that "game rules are not copyrightable," but who wants to be the first test case? One could get around it all by making up new terms like "Fancy Class" or some such, but that begins to feel silly.