Fair point, however if you place your foot on the inside of his knee and push "down and outward" while you have hold of his head in a clinch or push his kneecap down into his sock hes not getting up anytime soon, and wont work or walk properly again without the help of an orthopedic surgeon. Especially if you have strong legs, but just your weight should be enough. Its harsh, but if its a group or an armed attacker in an attempt to rob or assault me Ill let em have it. Keep in mind it may land you in jail in Australia - New Zealand is a different story.
Back on topic, while the arms and chest do contribute to punching power, your back in particular the lats provide huge punching power. However, if you can incorporate the whole body into the punch, legs CORE torso then arms the damage will be devastating.