In an ideal world this is true ... but we don't live in an ideal world, and not every diver is going to react to a "relatively minor emergency situation" in a calm, rational manner ... even if they have been trained to do so. Because not everyone practices what they were trained to do, and therefore they may not think to fall back on training (even good training) during a moment of stress. And because "training" often refers to the act of mimicking an instructor once, while on your knees, in a planned exercise, which is not at all how things happen in the real world.
Sorry, nope ... can't accept that response. Not being able to breathe once submerged isn't what I'd call a "relatively minor emergency situation" ... it's just about the only emergency situation I can think of where your life is being measured in seconds unless you do the right thing to remedy the situation. And the right thing in this case is to reach behind your head and turn your air on.
... Bob (Grateful Diver)