In the world we live in it is very possible for a country to try and nuke us. There must be some form of protection against enemy missiles. Nukes are real and very dangerous weapons of mass destruction. There are currently 9 countries in the world ( including the U.S. ) that have nuclear weapons and I would not be surprised if terrorist organizations had them as well. Maybe its just Paranoia but we must be able to protect ourselves since anything can happen.
The most visible treat would be North Korea. The temper of North Korea is unpredictable even to countries like China. They sometimes are very aggressive and shoot directly towards the South on some occasions. What are we supposed to do? Sometimes North Korea just forgot who themselves are so we warn this country by deploying defence systems right at its front door. By doing that, we wish this country could perhaps think about consequences before becoming stupid. Allies around the Korea peninsula may get a little bit sense of safety by showing muscles. Moreover if what unexpected has happened, it would the most efficient way to destroy ballistic missiles when it was soaring. Hence US security is improved by deploying antiballistic missiles in the front line.
I am in favour of extending defence systems to the very front line of potential treats like China and Russia. Those missiles could not be used to attack any country, meanwhile they ensure a better security condition for another group of countries in the rest of the world. It's a Pareto Improvement so why don't we do this?
Why do you think countries like North Korea are building ballistic missiles? Because they are relatively cheap. To built a military capable of projecting force is an enormous expense. You need an Air Force with strategic bombers and fighters to protect them, you need ground forces to seize and retain land, and a navy to supply the whole damn lot over distances. To build that to the point of credibility is insanely expensive.
If you are spoor country? Why not just build some crappy rockets and strap a nut or two onto them? Suddenly you are force to be reckoned with. You can behave in outrageous manners because no one wants to risk a nuclear attack to stop your misbehavior.
So what happens when you develop the capability to shoot down those rockets? Right, places like North Korea are then just two bit little piss ant dictatorships again.
The government already spends an obscene amount of money on defense and rarely on domestic infrastructure. Because of our lack of economic focus, our economic position has dwindled. Military expenditures mostly lead to things in foreign countries that cause destruction. Estimates for the missile defense system have been high, and it would be better for America to go the opposite direction.