I agree. But, for me to feel like I'm actually doing the setting justice and running Forgotten Realms and not just "Hussar's loosely inspired Forgotten Realms" game, I'd want to use the details. And, again, if all I'm using is the FRCS (or whatever), well, that's barely a FR game, AFAIC. Sure, you've got some of the proper nouns in there, but, without the rest of it, it's not really a FR game.
It's kinda like Dragonlance. If you want me to run a Heroes of the Lance era Dragonlance game, I'm all over that. I can do that and I think I could really do it justice. But, set it in Age of Mortals? I'm completely lost. Totally out to sea. Recent discussions about canon have highlighted quite well just how out of touch I am. So, in order to run an Age of Mortals game, I'd have to do a LOT of homework. Several supplements and probably a few novels as well worth of homework. And I'm just not interested in doing that much homework in building the background of the campaign. I'd rather just build the campaign instead.
Again, and I keep asking the question, where is the upside for me to run a Forgotten Realms game that only references the FRCS (to use that example)? What am I getting out of it that I can't get anywhere else? Isn't a large part of the draw for FR, Realmslore? It's not like the setting is terribly unique in other ways. It doesn't have the built in feel of something like Ravenloft or Al Quadim. It's not drawing on a bunch of new(ish) ideas like Planescape or Spelljammer. It's vanilla D&D. I certainly don't need a setting guide to run vanilla D&D. I can do that on my own pretty darn well.