So the Iyanden codex supplement is out and according to Natfka over at Faeit212 (welcome back BTW) it's a winner. I know that some see these as just another money grab, "Why isn't it just in the codex" etc. However, I do see pro's to this as well. For example, we're now hearing that Codex: Space Marines will have a similar set up. Consider that in the current C:SM, there's very little to flavor the the various chapters besides the various SC's which don't really hit all of the chapters (Iron Hands, anyone?). I'd love to see something like the chapter traits from the 4th edition codex albeit not as abusable (i.e. drawbacks that really weren't). So the codex supplements would be a way of bringing that sort of flavor back in, without forcing someone who wants to do Salamanders to pay for all of the supplementary material for Imperial Fists, Ravenguard, White Scars.... you get the picture.
So where does this take us for some of the other Codices? How about Imperial Guard? I'd love to see the return of regimental doctrines, but absent that, supplements could accomplish the same thing. The basic codex would probably be Cadians, with supplements for Catachans, Steel Legions, Valhallans, Mordians, Elysians(?), and who knows what else, maybe even Armored Company (dare to dream).
To say nothing about the various clans of Orks. Like wise the Codex:Chaos SM could be expanded via supplements to actually depict the various legions (Iron Warriors, etc).
Anyway that's my 2 cents