M.A.D. Mutually Assured Destruction. If they have missiles and we don't whoever 'they' happen to be can blackmail us into doing whatever 'they' want. If we all have missiles and counter missiles then no one will use them because it will be detected and a counter strike will be enacted: we all go boom and no one wins
Think of the damage that could cause. Without this we are defenseless from any thing from a cruse missile to a nuke. The cost may be high but If we build one of these missile defense systems in every city or urban area then we won't half to worry about an impending nuke.
The United States should not develop and build a new missile defense system. As of yet, we currently have a fully functional missile defense system, and another one would not be beneficial. Firstly, the cost of developing and producing a fully functioning missile defense system can range within the millions. Our economy does not have the financial leniency to afford a new system.