Commercial Diving Perspective
While I was working in the Gulf of Mexico as a commercial diver, we NEVER dived with valves up. Why?
First, the valve down position allows the tank to be turned on or off by the diver. Typically, a bailout bottle is left in the off position until it is needed.
Second, to protect the valve on the tanks from debris from above.
When I was back in commercial diving training,, one of my instructors who was a saturation diver for many years was asked where the valve up thing came in Scuba.
His response was that diving valve up allows use of a standardized tank size by divers with a range of different heights. Hence, an aluminum 80 can be used by a 5'4" female or a 6'2" male and the tank valve will be in same relative position behind a diver's shoulders. This allows a dive ship or dive operator to have one standard size tank for all. In addition, as you mentioned, in the olden days of scuba, the double hose regs needed to be positioned behind the head to function. When the double hose went away, no one saw a need to change.
As far as comments that diving valve down exposes the valve to greater risk. Commercial diving exposes equipment to considerably more abuse then scuba will. Banging into steel structures like oil platforms, ship hulls or diving bell hatches as the diver climbs in and out. Given the stringent safety laws and regs that commercial diving adheres to, if diving valve up decreased the risk of valve damage, tanks would be dived valve up.
As far as comments that diving valve down exposes the tank to damage when taking if off. When you remove the tank and harness, you remove one arm and swing the tank down with other arm. In my experience, I never once banged a valve taking it off.
In practice, the tank is secured with giant hose clamps to a rubber backplate riveted into a commercial harness. Link to see a commercial harness below -
http://www.millerdiving.com/harnes.html
(they kinda look like a backplate / wing setup minus the wings,,,dont they?)
In use, the regulator hose which is equipped with a quick disconnect is run up the side of the tank connecting to the bailout valve on a helmet. This keeps the hose streamlined with the body and not bowed out behind the divers head.
As a note, commercial divers will use bail out bottles ranging from a single 50 to double 120's depending on the dive.
I personally still dive valve down in standard commercial fashion with the only addition being stanless steel valve guards to set the tank on when on the boat in the tank rack.
