Legion is Sanderson's Metaphor for Himself

[Alfa he/him]

In the foreword, Sanderson writes that "Legion" was a very personal experience for him. And, in a way, each writer is not unlike Legion.

I think, that having such a vivid imagination as Sanderson inevitably leads you to think like your characters. Note, that Sanderson is (by most definitions) a completely sane person. However, a fair bit of his characters are slightly insane (think Kelsier, Wayne), have strong mental problems (think Kaladin, Shallan etc ad infinitum...) or quirks (notably anybody); very much akin to the Aspects.

And I think that having all the characters brawling for attention in his head might be taxing... And it might be also lead to questions, how strong your characters influence you. Might it be, that a certain aggressiveness comes from Kelsier, or that depressing thoughts from Kaladin?

So, the Legion books are about the way of the writer to deal with this...

[The Invested Beard]

I just wrote basically the same thing in my reaction post. I think you've hit the nail on the head.