In my humble opinion, allowing ducks to settle in your decoys before spooking them for easy shots is unsporting behavior. The duck is at an extreme disadvantage in that situation. Personally, I would not be comfortable practicing that as a method of hunting.
The way most hunt ducks is to attract flocks with decoys and calls and when in range, shooting. This provides for split second decisions to be made and shots that vary between easy to impossibly hard. The other method is jump shooting, which basically involves hunting ducks like upland birds, flushing them as closely as possible. This provides a challange B/c it's extremely hard to sneak up on ducks.
But there isn't a challenge in spooking ducks off your decoys after they land. The ducks have at that point become "Sitting ducks," with little challenge to the hunter. I'm not a big fan of duck meat anyway, but I just don't think it's worth having more birds in the bag if you have to pretty much trap the birds on the water. Personally, I don't need that much time to decide what's a drake/hen.. that is what hunting is all about.
"sky busting" is the practice of shooting flying ducks that are nearly or out of gun range. Of course that is unethical and there isn't anyone that would say it was effective at bringing down any bird. It's just a problem that the inexperienced and frustrated can't help but do from time to time.