Remember this Dev Watercooler? It was said there, that "Unlocking flying via mastering content on a ground on one character - is general blueprint going forward for content to come". Don't you think, that no flying on Argus breaks this promise?
I want to clarify one thing. Biggest question isn't why Argus as it is now doesn't have flying. Answer is obvious - because it's designed without flying in mind. Biggest question is why it has been designed without flying in mind in a first place, after it had been promised, that flying would be granted for mastering ground content on one character in all future content? I.e. we don't actually ask Blizzard to enable flying on Argus, cuz it's design is so bad, that flying wouldn't help anyway. We ask them to never make such no-flying only locations again.
Another thing - about no-flying "isles" in old content. Rules of obtaining flying have changed since 6.2 - therefore all other "rules" don't apply too. That's why it was so silly to hear, how Blizzard used this "It was like that back in <xpack>" argument.
About "Argus was added in patch, therefore no flying" argument. Ehhh. I've forgotten, that I've been sitting on official forums all that time and that this argument is proved to be wrong there, but not here, I guess. Don't you realize, how silly this excuse from Blizzard is? You should clearly understand, that whole xpack's content is most likely already developed by the time, when xpack is released, and that it's only up to Blizzard to decide, whether they want to spoil some parts of it via showing them in Beta and some scenarios or just hide them from players till some future patch to make them surprise. Blizzard just can't show whole story of xpack right at it's release, right? And basing whole flying/no flying decision just on such crappy thing, as whether location (but not content itself and location is just a bunch of textures) is shown prior to it's release and reused later or not - is just silly logic. This two things just have nothing to do with each other.
Also. Only BC and MOP had no-flying content in last patches of xpack. All other xpacks had them in a middle of xpack - and therefore such content could have been simply skipped without any harm for culmination of xpack's story and progression of your character.