Reach
Most people can reach much more comfortably behind the small of their back than they can behind their shoulder blades, even more so when they are wearing 7mm of neoprene or a drysuit, and if they have to use both hands at the same time. It's as easy for your buddy either way up.
Hose routing,
Yes the hoses have to be slightly longer for the run up under your arm, however they are kept naturally close to your body and much less likely to get caught on an obstruction. If they do get caught then they are in front of you so you can see what is happening and how to resolve it. The exception to this is the normal inflator which would have a worse run. However there is no reason why your inflator must come over your left shoulder it could, just as easily come round your waist, resolving that problem. With this routing you would be able to see your first stage by just pushing your rig slightly to one side and looking under your arm.
Entanglement
You are normally swimming forwards, and your first stages act as a set of hooks to entrap you when you run into an obstruction. With the tank upside down there is nothing for the entanglement to catch on when you just reversing backwards, it just slips of the end of the tank. Yes I know some people swim backwards sometimes but that is the exception.
Reg Recovery
Why would your long hose be hanging off your right shoulder? Much more likely that it has fallen behind you, caught under your first stage and hanging halfway down your left side, trapped under your wing. Why because thats where Murphy decided to put it when he decided to screw your dive up.
Upside down tanks are, without a doubt, more liable to damage when you are putting your gear on and that may be the main reason. However I am beginning to think this is like din versus clamp, everyone knows din is better but momentum keeps the clamp going.
