McDonalds quality control is second to none. Their standard menu tastes practically the same everywhere in the world. It's the same with Starbucks. I know that no matter what part of the planet I am at, I can get exactly what I expect to get. I'm never disappointed. I can't stress how important that part is. Bad quality control can really injure restaurants.
Perfect consistency is only useful if your perfectly consistent product is any good. Much depends on your target audience. Bad quality control can hurt a lot, but so can perfect consistency in producing a bad product. If your audience is the sort that thinks McDonald's is quality food then that's obviously where you should aim, but many audiences don't.
Having worked in Quality Control in a major multinational, and studied and lecture statistics, I have seen that significant decreases in quality are accepted in exchange for increased consistency.
Quality is the measure of how closely a finished product meets its design spec. McDonald's products are insanely high quality, despite not being very good - a Big Mac was never designed to be healthy and satisfying in the first place!
A relevant example of this is organizations who are CMM Level 5 certified.
