I found that reveal to be a rather natural one. Most of the organisation members showed a behavior that pointed to feelings of some sort. Demyx was a coward. Saix was angry. Axel longed for companionship. Larxene liked to hurt people - which she wouldn't enjoy if she didn't feel something in return. Some people claim that the plot point is stupid because - how wouldn't you know that you can feel? - and I can understand them, but feel differently. Humans are beings that are naturally capable of feeling and still we often don't really know what we feel. Am I really in love? What is this feeling if I see something that hurts me - sadness? anger? disappointment? - we have complex emotions that we sometimes deny according to the circumstances. When I try to remember things from my childhood, it feels blurry ... I can remember the situation butoften enough can't wrap my mind around how I have felt and, as a result, think of this logically instead of emotionally - "I have probably felt sad". If someone was constantly telling me I can't feel and if I might feel that I can feel, it's just pretending, then I would probably believe that. Given the circumstances that the nobodies only vaguely remember their former lives and try to remember what it feels to feel in a rational way, it's no amazement they thought they were just pretending.
I could emphasize a lot with that. With them and their struggle trying to be a whole person and, in not knowing how to do that, following orders.
I also liked the 13 Xehanorts plot twist because it made sense to me - and, unlike some other plot twists, had some hints here and there. What I disliked about it was that it stripped the members of their identity and the result was quite inconsequent - in cases like Terranort Xehanort had complete control while in other cases like Saix he seemingly couldn't care less about their doings. I would've loved to see more of a struggle there, more original motives (like "I want a keyblade") instead of saying "you've been norted, you are part of the enemy now, you don't need a personal reason to fight". So, in the end, I like the concept of 13 Xehanorts because it made sense, but disliked the execution because it contributed to the members not having much character as individuals.