As a teacher, I see exactly what these instructors are trying to do. I like my Serpas, but it's their class, and they are the ones taking responsibility for everyone in it.
I guess what I don't get about the "fraction of an inch change of drawstroke" is how these people are drawing when they don't have the Serpa button to operate. Are they curling their trigger fingers and pressing them against the holster then? Probably not, right?
Are they instead putting all four fingers on the grip, thus not taking a firing grip until the gun is out of the holster? Probably not that, either.
If they know anything at all, they're probably drawing from those other holsters with their index fingers extended straight along the holster/gun. That is the standard taught everywhere as far as I know.
If you just do exactly that with a Serpa, you should be fine. The key in my mind was to forget about trying to operate a button and simply draw the pistol the way I always do, with the trigger finger out straight for safety. Maybe others don't touch the holster with their trigger fingers when they do that; I always have.
On the other hand, failures locking guns into the holster are a bigger problem. I only use mine for USPSA and range time, so I haven't had that issue. But I remember when Brigid's failed on her--and it was clean.