I'd submit to you that a guy that will negligently discharge his pistol with a Serpa holster will do it with any holster.
I see your point, but it's the Serpa, not "any holster" that seems to be the common denominator with re: to AD's. Ok, ND's.
That's not the only reason. Farnum won't allow them in his class and, if I'm not mistaken, Gabe won't either.
The reason Farnum gave is that he's had too many reports of holster jamming up so they wouldn't release. A report, I believe from Iraq, had a very small twig jam up a holster--- preventing the weapon from being drawn.