Pfffft, like those three have the whole picture. And who wrote down all that, hmmmmmm?
Of course they don't have the whole picture. They can't even agree with each other. But it still provides a more balanced viewpoint. And if, say, Kanako tells us why she chose Utsuho to be Yatagarasu's vessel then we suddenly have a more informed viewpoint on the matter. Is she 100% factually correct on every issue? Probably not, but no one is, not even in real life. As for Akyuu being the secretary, why would she go out of her way to include stuff that contradicts what she writes in the other sections? Not to mention the fact that there are at least 4 (5?) people who would be able to instantly call her out on it.
Basically, I'm tired of this whole "Akyuu is a lying liar so I can claim that whatever random thing I make up is just as true as the canon side materials". Yes, Akyuu is biased and often ill-informed. However, that's usually made extremely obvious in the text. That doesn't mean that everything she says is completely wrong, or that we should just completely ignore the book. People are imperfect, even historians. Question everything you hear, look at any fact from multiple angles. Sure, those are good habits. But honestly that kind of thing is symposium's specialty anyway. It
does show multiple perspectives on things, by presenting a dialog between multiple opposing parties. What was that term... dialectic? The truth is revealed by disagreement.
Edit: Of course, I have nothing against people telling stories that directly contradict canon. Heck, most of them do in some way or another, and I'm certainly guilty of thinking of characters in non-canon ways. I just don't like it when people try to dismiss what little canon we have as meaningless. Or worse, never read it in the first place. It's one of those "you have to know the rules before you can choose to break them" things. That's a pet peeve of mine.