The primary goal of the nations missle defense systems is not ICBM's, here is an excert that might help out:

"The primary mission of National Missile Defense is defense of the United States (all 50 states) against a threat of a limited strategic ballistic missile attack from a rogue nation. Such a system would also provide some capability against a small accidental or unauthorized launch of strategic ballistic missiles from more nuclear capable states"

Would you agree that there is a "possibility" of an unauthorized launch from a country with the stability of Pakistan, or India, or Iran, how 'bout North Korea?

And while agricultural subsidies do aid farmers, even the big ones, it's main purpose is to help the american consumer.