THAAD's are not cheap. You've got to know that the missile is going to hit something valuable before you spend $ millions to shoot it down. If it is going to fall harmlessly in the Pacific, do you want to waste a few mil shooting it down?So are we afraid to try and shoot them down and miss?
Plus, if you shoot and miss, for any reason, how will that fact influence North Korean behavior in the future? How would it influence South Korean behavior? Japanese? Chinese?