It's kinda funny how people think a BP/wing is strictly DIR. "You can only use one if your trained to use it".
As far as open water divers are concerned, There is no difference between it, And a jacket BC.
You got your inflator, And your dump valve. You put the tanks on your back.
Wow, That's pretty complicated. I think I need special training by GUE.
Honestly, The only thing it changes, Is a little wight off your belt.
There is nothing wrong with a NAUI instructor using one while teaching open water students. The only problem would be is if the LDS he or she was training through did not sell that particular piece of kit. You wouldn't want your instructors sending students money elsewhere would you?
Also, A lot of the "DIR" philosophy was around long before there ever was GUE/DIR/Halcyon.
After all, Al Gore did invent the internet, You know