Of course the anti-ballistics will not provide blanket protection for Israel, nor does anyone in the military think it will. But it can help minimize the initial blow of a missile salvo, and give time for a devastating response aimed at missile launchers. This, in itself, provides deterrence value. Furthermore, if you ever want to develop a new generation of more effective anti-ballistic defense options, you have to "waste" the money now and build the less effective systems. Even if peace in the Middle East is the best Israel's long term security, it must continue to seek ways of protecting itself from the very real threat of it's neighbors.
13 security personnel killed in Iraq suicide bombing (DPA)
from the article: Missile defense systems won't protect Israel