As tensions grow in Russia and the EU continues to add nations, missile defenses in the Czech Republic and Poland are not only justified but wise in providing a front line defense against Russia should tensions move into Eastern Europe. Without the benefit of missile defenses, there would be virtually nothing from stopping Russia from deciding to re-sovietize.
A missile defense system that could intercept nuclear warheads and neutralize them has been a dream of military strategists since at least the 1980's. President Reagan made it a priority with his so-called "Star Wars" initiative. This type of system is so appealing because if one nation had the technology they would have the ability to unilaterally launch nuclear strikes with out fear of repurcussions.
The problem is that the technology doesn't work. Many, many attempts have been made to develop reliable missile defense systems and none of those attempts have worked. There are systems, such as PATRIOT missiles, but they have limitations.
It just doesn't make sense to deploy missile defense systems in Poland or the Czech Republic when the technology is still unreliable. The security we would gain from these systems is far outweighed by the danger caused from angering Russia. The gain is not worth the cost.