Ah, I see what you mean now. The soul serves exactly no purpose at all in the body except allowing it to toggle its functions "on", as it is merely a condition to make life possible in the first place. Nothing in the body's functions requires magical energy, and left to its own devices the body would consume no magical energy on its own, either (the mind would passively waste small amounts, but not the body). Rather than trying to make sense of the soul as an energy that exists within the body, it would probably be more sensible to view it as a second body within the body; an incorporeal copy of it, one could say, that simply happens to occupy the same space in the Planes as one's physical body does. The soul can exist and function without a body, as seen upon death, undeath and dreaming, whereas the body can only be alive when occupied by a soul; thus injuring the body will usually not harm the soul, but when the soul is weakened the body... well, in lack of a better way to describe it I'd say that it tries to alter its own condition to match that of its soul in order to remain compatible with it. Careful usage of magical energy will thus only leave the body feeling weak and fatigued, but not actually do any damage to it, but recklessly spending it will start to leave the soul "incomplete", which the body responds to by slowly disassembling itself.
...Or something like that, I think.