No. The only way you're going to see what Character A (such as an Organization member) "really" is/was is through Character A, not indirectly through a Character B like Xion. You see what they say, not what others say about them.
One could argue that Xion might be the situation through which we see the characters of the other Organization members. For instance, the actions of Xion might compel one member to do one thing while another does something else, which would implicate certain things about their own motives, desires, etc.
But that's all wrong. First and foremost, Xion is not needed for that. You don't need a new filler character to create scenarios in which character development is seen. Just the fact that there are 13 members interacting with a supposedly common plan is enough. And second, that is not the route they are taking with Xion anyway. It should be clear that they aren't putting her on the back burners, she's going to be one of, if not the, most prominent character in the game. We're going to be finding out "Who Xion is" and we can see that a lot of the emphasis is going to go towards her struggle to answer that. So, undue emphasis will be placed on her, not on the other Organization members who clearly need it more.