All of Europe needs a missile defense system against Russia, especially Poland. Russia is showing new signs of aggression under president Putin. Europe needs to develop new ways to defend itself in case of a war. The existence of a missile defense shield could change policy makers abilities to deal with Russia.
The usefulness of a missile defense system is questionable at best. Similar programs in place in North America have consistently shown lackluster results despite the billions of dollars invested, and there is no reason to expect a system deployed in Eastern Europe to be any more effective. Even if one could guarantee that the system would be effective, likely targets for missile strikes are not in Europe. Nations in the Middle East that are hostile to western interests do not have the capability to launch missiles at targets in Europe; more likely targets, should hostilities occur, are located in the eastern Mediterranean and in particular Israel- regions that the proposed missile shield would not cover.
Finally, it is clear that deploying such a missile shield would drastically damage the relationship between Western powers and the Russian Federation, which feels that the true motive of deploying such a shield is to hamstring its ability to deploy its nuclear deterrent. Assuming the missile defense system was demonstrably effective and also addressed true threats to the safety of people in Europe, this may be an acceptable trade-off. However, as already discussed, this is not the case. Further dampening relations with Russia over a failed system that attempts to address a non-existent threat is not favorable.