I know I am a noob, but you are all wrong! I have talked to a few fighters, amateur and professional (although none in the UFC), and all say the same thing! It is just not acceptable behavior to intentionally end a career. The side kick is well respected by many MMA fighters but is well known to cause injury when launched at the knee (a JKD practioners first thought). Intentionally disabling another fighter is considered rude and unnacceptable and will usually end in other fighters planning on doing the same to you. Fighters avoid this for the same reason you don't sneak in a below the belt punch in boxing when the ref isn't looking - it just isn't cool.