Do you object to the notion of identifying who had a good start and who did not? The threshold has to be set somewhere. And pitchers aren't robots, so nobody is compiling cheap QS, game after game, at exactly 6.0/3. The guys who lead the majors at the moment in QS have names like Kluber, Verlander, Cole, and Scherzer - no bad pitchers are sneaking in as QS league leaders, just under the wire. It's a useful quick-and-dirty metric, and indicates who is giving their team a chance to win, arguably better than the venerable W/L stat. Choose a tighter threshold, and you'll be throwing out a lot of starts where the guy's team won without scoring a ton themselves.