National security is more important than the economy, social welfare, the transportation network, or even keeping public utilities up to date. All of those things cannot be maintained or improved if we have poor national security. 9-11 was the most expensive disaster to ever befall America, and that is just a taste of enemies want to do to America.
The missile defense network is one major thing that we can do to protect out country from known threats. Secondly, it is a deterrent to countries developing missiles. If they can't hit us (and our allies), why make them. Countries wanting to blow stuff up are going to have to come up with new riskier ways to blow things up.
When it is easier to get a nuke than it is to feed people in your own country. That is a major nation security problem, which needs to be handled. Since we can't nuke everyone, or get other countries to put down their arms peacefully. Our only option is to protect ourselves.
Since missile defense is a tougher technology to develop than nuclear weapons. We are going to have something that other countries want have until they figure it out. Secondly, we don’t need this technology now, but we might need it in the future. It is a lot easier to develop something like this of a technical nature when you back is not against the wall and you have time to test it and get it right.
The main technical problem with a missile defense shield, is that you can't hit anything you can see. As it stands know, America is really the only country to developed stealth technology, although that won't last long if countries need missiles that can't be seen. Russia is going the wrong direction in trying to build missiles that out maneuver other missiles. Our lethal combination of stealth technology and defense shield will give us maximum protection.