Okay, come on guys. I don't even know where to start in response. Are you seriously suggesting that if a diver makes a conscious choice to button purge as their routine, be it a pony reg, alternate air or even a lost and retrieved primary that it is a lack of training or poor technique or otherwise wrong in some way!? Give me a break. Thats ridiculous.
I guess if your lung capacity is such that you are confident that you will always have the lung volume you need if a panicked diver grabs your primary (has not happened to me) or another diver decides to occupy the same space as you and accidentally kicks your regulator from your mouth (has happened to me) then by all means, practice orally purging only. But I CHOOSE to practice button purge as the safest option for me. And since I like consistency, I use it with pony, alternate and primary. Yes I am familiar with the cautious first draw and repurge until the reg is clear method and have used it but I CHOOSE to use button purge AS A BETTER OPTION FOR ME.
in regards to leaving the air on or prime and cutting off, I have seen both sides argued in past threads and no consistent answer. I choose to leave it on knowing it is where any leak or free flow would be obvious. Cutting the valve off would arguably increase the risk to the first stage if pressure is lost but it would seem to be the same risk as soaking the regulator with dust cap and unpressurized and we all are familiar with that debate as well. I have also seen in in line shutoff valve that could be used to eliminate any risk of flood but as others have pointed out, it's another possible failure point. Whichever you choose, again, be consistent.
And Happy Diving...
Lisa