Look at the videos, in a combat situation you cannot unsheathe your sword quickly enough if you carries on the back (and even if you can it is not an easy thing to do, I have prove it myself). If you're fighting you want have your weapons ready to use. It's a question of economy of movements. If you can use 1 second you don't want to use 5.
In short, having a sword in the back is dumb, having it in the hip is smart. If a dûnyain is indeed smart he will carry a sword on the hip, where he can use it quickly and deadly.