I also like to think that spirits and demons as they are aren't powerful until when confronted with something from the physical world. For example, I think Pride demons would truly manifest their characteristics only when a person has communicated with it and that person would act as like a mirror of itself. I mean, if there weren't mages running around tempting magical beings with their mere existence, what would those demons/spirits be doing? ... In contrast, it seems as though the Gods and Espers of Ivalice seem to maintain a kind of individuality as well as being able to exist in the mortal realm without the aid of humans pulling them out.

That is a really interesting point, and I will have to think about the implications! Hmm. I also like the idea that Mist is "native" to Ivalice in a way that the Fade is not.