The reason that you don't do this is because unless the cylinder stop is fully depressed, you'll end up with it dragging against the cylinder, leaving a drag mark around the circumference of the cylinder. With single action revolvers, there is a half-cock notch, and when the hammer is in the half-cock position, the cylinder stop is fully depressed. Double action revolvers, however, have no half-cock notch, so when you just pull the hammer back part way, you really have no assurance that the cylinder stop is fully depressed, and may very well end up with a drag mark around the cylinder. A drag mark like that usually indicates that the gun is not timed properly or that it has been handled by an idiot.