Also i can see a lot of potential confusion with an inline shut off valve. What if the diver charges his reg, trurns the tank valve off and then has the inline valve open.. and the air leaks out and he tries to use the reg? He is going to flip the inline switch and get nothing...
Then he is going to flip the switch back and try again to suck.. now the in-line is off.
Then he is going to think oh crap, i must have forgot to turn on my pony bottle, so he open the tank valve and tries to suck... again NOTHING - (because the inline is off).. Now he's confused.. and air starved..
So he thinks oh maybe I turned the tank valve the wrong way, so he shuts the tank off....
The whole scenario is way too confusing...