To your point, there are some characteristic flavors shared by the best of the style (Grande Reserve, ABT12, Rochefort 8, etc.) but definitely some deltas between them as well. It's a little like brothers that look alike but have personalities that are different enough to warrant the comment, "Funny they are in the same family...."
Good point, but I do find it interesting that in the 2015 guidelines they have retained both the Belgian Golden Strong and the Tripel styles even though the description for the BGS includes the words "Strongly resembles a Tripel...". If I'm at a bar and order a Belgian Golden Strong or a Tripel, I have a pretty good idea of what I'll get within a fairly narrow range of flavors split over two categories where the BGS may have a lighter body. If I order a BDS, then I need to take a sip before I can tell whether the beer is a Rochefort style BDS or a Westy/St B style BDS. In the case of brewing a BDS, then a brewer could decide up front where in the spectrum they're shooting for and use that decision to drive their choice, levels or exclusion of the specialty malts.
It just seems to me that if Specialty IPA can be broken into six sub-categories, then BDS could at least be broken into two...