Thorsim-
Poor choice of video to support your theory.
Beyond the already mentioned aspects, I'd like to add that those planes weren't representing the way the AH planes are being flown in the game instances you refer to. Near stall speed, flaps fully deployed, throttle set low, intending to land is different than slow, full throttle, with flaps deployed, with no intention to land.
Getting caught in a suddenly life-threatening situation, that you have probably never experienced and now only get one try to survive, is different than trying to cut inside a cartoon plane for a shot...
In your response to colmbo, you give the two possible causes for those crashes as lack of control, or pilot "suckage". Awful narrow-minded... Could it have been anything else? Could it have been pilot fatigue, illness, distraction, obstructed view (which was the explanation given for the P51 crash here in Oshkosh, unless that's been amended?) lack of experience, damage to the plane, etc?
Planes like that are and were landed quite commonly with full flaps. Normally, it didn't result in a crash. The vast majority of the time everything turned out all right... The flaps were designed to "help" at those speeds, not create a way to kill the pilot.
RC is a bit different I suppose, but doing barrel rolls with full flaps doesn't destroy my plane, or make me crash... Of course, I'm not trying to fly inverted and maintain altitude then, but I'm not in the game (rolling scissors), either...
I fly a "flap-adjustable plane" in AH, almost every single time I fly. Going full flaps is not a great thing. They create a huge amount of drag, and make it very difficult for me to control exactly where I point my nose/guns. I'm using 20 degrees or less the vast majority of the time. When I deploy more, it's brief, and out of desperation or an "all or nothing" shot opportunity, ESPECIALLY if I'm low. My plane stalls around 76mph, and the full 50 degrees drop at 150mph, in case that question pops up.
I tend to agree with you, that excessive flap use is "normal" in AH. But as others mentioned, the "higher-skilled" crowd don't use them that way. That would seem to show that there may be an advantage to not using them as most folks do in the game...
As a trainer, I seldom see folks come to me who are using flaps "optimally". They're either not using them at all, or over-using them, or using them at the wrong times, in the wrong situations, etc. That type of use gets them killed, even though it may be from bullets instead of hitting the ground.