I personally don't understand where people keep getting the theory that the Soul can create it's own sentient being.
The Heart, in simple terms, defines you as a being (it is your identity). The Body gives you a physical shape, and allows one to move around.
We know from Nobodies that the Soul just allows the Body to remain animated, right? Even if that's incorrect, the soul still doesn't do much at all. If a Soul was separated from a Body, it would float around and be unable to do anything. If a soul was able to assimilate a dark being's shape, there wouldn't have been many souls floating around in The Underworld, right?
Without a Body, a Soul cannot do anything. That was always my understanding, so if someone could prove me wrong, maybe I could understand the point of a soul. To me, souls=Unbirths just doesn't ring.