Amos, I don't entirely rule out the existence of duality. I'm simply asserting that it's something we have no way of proving or disproving, which makes it practically useless. In fact it's worse than useless, because an experience might be ascribed to the "spirit" as an explanation when a physical explanation is actually correct. But because the "spirit" explanation exists, it discourages anyone looking for another explanation - after all, we already have one, right? That's how we get to the "just because" point. Actually it's not "just because", it's that it's "spirit", but for all practical purposes the two are the same - it's saying "spirits just do that" and that's the explanation.
The liability of electing one cause over the other incorrectly is a two way street. The strong conviction in the fundamental of matter and structure as the provenance of thought is just as corrosive as the strong conviction that consciousness is the ground that matter appears on, as a transient figure. It is not the case that the relationship between mind, spirit and body cannot be tested in general. But it is difficult to test it by physical lab standards which is frustrating to some who feel such standards are identical to "good scientific process". But the two are not identical by any means.
As for saying "spirits just do that" I would disdain such a claim as an argument in explaining phenomena just as roundly as I would reject the argument "It's complexity that acts that way, and the universe is complexity from there on down!" Because I insist that the phenomenology of consciousness be fully taken into account does not mean that it should be considered a deus ex machine for explaining things. That would be silly.
There is of course the difficult fact that life makes decisions and generates intent. Good ole molecules behave much more tractably. They don't jump up and postulate new space or generate insight just when you are writing up your lab report. Consciousness does do such things, and that is one reason why well-trained physical scientists don't like to mess with it. I can sympathize that they feel they have "real" (meaning material-universe) work to do and wnat to get on with it.
But if serious scientific work were to be done in a framework that allowed for such phenomena, and ways were devised to at least measure them in a general way, or otherwise generate "data" from them, I think we could get a lot further in the fields where it is important, such as psychology and medicine. And if we got a lot further in them, the ripples of betterment in the madhouse of Western culture would probably be valuable, too.
The problem is though that if you want to figure out and fix depression, schizophrenia, chronic pain and all the other woes that a malfunctioning brain can dump on you, you'll need more than "it's the spirit". The most basic assumption to be made is that it *might* be possible to fix them, and since the spirit is untouchable, that *might* put these down to physical problems (otherwise it wouldn't be possible to fix them).
There's no reason spiritually oriented remedies cannot be brought to bear on ordinary mental conditions, and to the degree they were effective, they would militate for the truth of the model. In a sense all Rogerian, client-centered therapy is spiritual, in that it works on the premise that thought and communication is able to remediate the psychosomatic and thus change structure. This is a large subject and won't be covered in these few sentences. But I would urge you to consider the consequences, for example, of a person reporting that they left their body, wandered about for a while, and returned to it.
If they are told by authorities such as doctors that this was an illusion, and that in fact they are not a spiritual viewpoint but are just a biochemical cluster of great complexity, the strong possibility exists that their morale will sag dramatically and the spark of creative insigth they were beginning to discover wihtin themselves will snuff out. If it does not make sense to you why this would occur, spend some time observing children who are heavily invalidated by their peers or parents.
Anyway, I don't mean to wend on intemrinably, please forgive the length of this post. I could sum it up by referring to the dot and the line story. If you run into a three-dimensional post in a two-dimensional world, arguing that it cannot exist is probably your least profitable course.