Yeah right, MAD is a deterrent not a first strike weapon. Besides that, asteroids spend most of their time extremely far from Earth. If you launched some mega project to get out there and redirect one that wasn't already headed for us it would take you years of preparation and then years of execution time. Your enemy could easily see all of this activity and decide to just start a conventional war in the mean time to take you out and still have enough time to deflect your war asteroid. It's a dumb scifi plot device not a practical weapon.