With the proliferation of nuclear weapons and more countries developing the missile platforms with the ability to deliver either nuclear or conventional weapons it is in a country's best interests to invest in the development of missile defense capabilities. The development of effective missile defense is one of the best long-term defense investments a country can make.
There is still a lot of crazy dictators in the world. It would be nice to feel safe from a nuclear threat. It is a shame we have to waste our resources on such things, but that is just the world we live in. Hopefully, one day we will not have to.
There are too many countries with the USA in their sights, I think we should defend ourselves in any way possible on our own shores. We have only had two really major attacks on our soil, Pearl Harbor and 9/11. But as technology in other countries improve, so should our defenses. We were under prepared both times we were attacked and it caused thousands of lives.
We already have a missile defense system, and while it makes sense to keep it updated and upgraded as much as possible, the simple fact is that most of those funds go purely to enrich the already rich defense contractors. Who, exactly, do we expect to use missiles to attack the United States? If the atom bombs finally do fall - there will not be any surviving it. Period. However, solving the underlying problems of the world such as starvation, inequality, ignorance - that is far more likely to save us in the long run. Instead of deciding HOW to fight - lets figure out how to make fighting unnecessary.
A missile defense system like Reagan's "Star Wars" initiative is as much a fantasy as its cinematic namesake. In order to fulfill its function, a missile defense system must be effective 100% of the time. If it lets even one nuclear missile get past, then it's a complete failure. Have you ever known a government initiative to be effective 100% of the time?