PTW read the thread before you post, it's been made very clear that IT IS NOT A DAMAGING strike. The fingers should be banned but i've heard the ref screaming at him to stop it against Vitor.
Of course its a damaging strike, the whole point of the kick is to hyperextend and damage the knee severly. Whether or not it's defendable by checking the kick is really not relevant, you can defend yourself from a nut shot also. Doesnt mean it should be allowed.