Another comment mentions training for center of mass, and I would assume that's what they do.
Let's say 80% of your shots constitute a "tight grouping", 20% being somewhere other than where you were aiming. Aiming for the middle of the body would likely mean you have a very high success rate of hitting your target individual. Aiming for the shoulder/leg/arm (each one harder than the last to hit) you are looking at a very low success rate of hitting your target. If you choose to use your weapon, I would hope you are confident you will hit your target and only your target.