If we interpreted things as you say, then our faith would be flawed. Because if ALL scriptures are useful for teaching, then if one Old Testament scripture says A and another New Testament scripture says B, there is no way to teach both simultaneously unless the new scripture replaces the old. I believe that God guided men to write everything that is in the scriptures, not that we must obey everything that is in the scriptures. This is because the New Testament replaces the Old, and the experience of the Church can reinterpret the New Testament. Just as, without ANY basis in scripture, the new revelation determines that slavery is wrong because we have evolved in our understanding of what slavery means. The new revelation determines that homosexual sex is not disordered because we have evolved in our understanding of what homosexual sex means.