I was taught that a knee kick was just a short legged kick. Every point of your body that can be used defensively or offensively is a weapon. I always liked to kick because it was many times more powerful than the hand. I can kick in really tight situations, but there is a limit, and when that happens you shorten the kicking weapon by using the knee. If you can get any part of the upper body close enough you can kick it or slam it with a knee. The knee is like an elbow. It is not as proficient as a fist or foot, but it can be used with good effect if it is what you have. You can hold on to someone and still pound him in the head with an elbow. The knee or elbow is simply not a primary weapon and any techniques that apply it like a primary weapon is a waste. It is a secondary weapon that is applied when nothing else is workable or if an opportunity actually presents itself.