I think that a National Missile Defense system is a good thing to have around, both as a proving ground for new technology and a small deterrent to limited missile strikes. Another thing that I see that this is good for is it allows progress to be made on this technology. Do enough iterations of this, and it could become a Planetary Asteroid Defense system. Supposedly, asteroids/comets are what caused the extinction of the dinosaurs, and the Earth will always be in danger of these types of extraterrestial collisions. A PAD system would be some insurance and breathing room to allow humans to get off the Earth, maybe to a terraformed Mars or Venus, and then to Infinity, and BEYOND!!!