So make them all start at the same height, at the top of the spine? What exactly about top-to-bottom orientation do you think precludes proper justification? You're still on about something orthogonal, the justification along the spine, which isn't affected by the overall orientation.

Also, someone posted photos of French bookshelves showing that their centers tend to be centered like ours are, so it's not any more readable. If you think of the spines as advertisements on bookstore shelves then this makes sense.