1) Not everything in those books are fiction. 2) For the most part we do not know how they are intended to be interpreted. 3) Spiritually is a nonsense word. If you disagree please define what you mean by it. 4) (depends on 3 ) Why does it make arguing about whether those texts are valid or not futile ? Even if they are texts that are open to interpretation we can argue over the validity of those interpretations.
However we do know that we don't have reliable sources to indicate who wrote or when most of the texts in those books were written, that these texts also have contradicting accounts of "historical events". Those books were also written in a time where people were even more gullible and uneducated than today.
In my opinion those books don't teach anything new ( to the time they were written ) or unique, as they are open to interpretation what actually happens is that people filter in what happens to coincide with what they already believe or project their own morals on those texts.
Example "Love thy neighbor". The word neighbor can be interpreted as fellow christian, so people from other beliefs are to be loved . Also it's more common interpretation is not unique to christianity nor the notion was originated by it.
Far from complete fiction, it seems to me that these books are written in metaphor and poetry such that the common man has an easy way of understanding. That doesn't mean they are fiction, it just means you can't take them literally.
For example the bible talks about the earth being held by pillars. Obviously this isn't literally true. But the earth is supported by a system of strong forces and what not which go far above the understanding of the common man. The bible means to say, 'there is something which keeps the earth from drifting into a black whole, into oblivion, or into mars'. It's just done in a way understandable to those who aren't physicists.
I think fiction is the wrong word. These stories, say the truth in someway. You have to read between the lines though, not literally. These stories tell you how people try to explain several social and natural phenomena, how they see and feel their surrounding and how they feel about their God. It is how these individual humans, which wrote these texts, experienced and interpretend things....they didn't lie or just invent something to entertain others, it is a truth amoung many others a reality how it was seen by these individual people. this is what makes these book so interesting to me. They can tell us how people though so many years ago.
also...everything on this earth, can be called some kind of fiction and not real. Since even science is bound to the human senses and interpretations. Things that are not able to be sensed by humans, are not experiencable for us. Even scientific instruments and measuring instruments are just an extension of our senses and numbers and units of measurement are just a try to interprete and cathegorise what we sense. in the end we don't know was it really real and what is just a fata Morgana of our senses. Truth and Reality are in the end just more likely theories, social concepts and results of indivual sensual experiences.
I agree with you, though calling them fiction is a bit too harsh in my opinion. They are very important works with great meaning. But they were written a long, long time ago by people from a different era. Taking works that old literally can be dangerous and will only lead to grief, as has been shown by countless wars and abuses with a 'holy word' to justify it. These works are important, yes, but they should be read as a historical work. Written by people from that time, with their morals and beliefs. The environment and era will always influence the writer.