Aeons are an interesting area. FFX introduces us to summons as playable battle party members, which also raises some implications about their nature. I expect to cover Aeons in far more detail later once we delve into their nature a bit more, but here are some thoughts so far.

Excluding the other party members in battle is obviously based on the fact that keeping them in would be unbalanced and completely destroy Yuna's functionality as a battle member. The narrative implication of this, though, is that Aeons are dangerous and Yuna needs concentration to focus attacks on the right enemies. This is not supported by the idea that she would summon Valefor in Besaid village though, so I'm not sure what to think.

That'll have to do for the moment as I got somewhere to be, but food for thought. If anybody has any other thoughts or suggestions, please drop them here. I'm curious to hear your ideas.