I considered doing that once. When sorted by subject/author you can end up with a mix of tall books and short books on each shelf in a bookcase, which means tha

I considered doing that once. When sorted by subject/author you can end up with a mix of tall books and short books on each shelf in a bookcase, which means that the vertical spacing of the shelves has to be enough to accommodate tall books on every shelf. That can waste a lot of space. Sort by height and you might be able to reduce shelf spacing enough to make room to add a shelf to the bookcase.