One thought regarding the "sword down your back" posture: all of the weapons instructors I've ever had, have said that it would be a very inefficient starting point for cutting with a blade; you would have to raise the sword up over your head from behind before cutting down with it. "Wasteful of energy."
However, on a battlefield in mixed melee, there might be an advantage to the position --
a) sword is safely against your back, allowing you to turn (tenkan) without entangling it in another's weapon;
b) sword is not "visible" to opponent in front of you, so you appear "open" as a target;
c) you appear exuberant and energetic, declaring that you have energy to burn, striking fear into your tiring opponents...