On the other hand, I recently read the proofs of a new work by a well-known literary writer. His suggestions were totally usable. For example, he insisted that each page contain 27, not 28 lines. He is equipped by constitution and information to be fully involved in this end of his work. Book design is a special and complex art in its own right.
Many writers have no eye for design.
Not every writer can carry a tune either. Then there's the time involved. I think I have a good eye, but I've picked up what I know about design through years of work. There's no way to take in quickly what I acquired over seven years as managing editor at Pocket Books. Simply staying out of what one isn't specifically trained for isn't a good enough answer, however.