If the incoming missiles are detonated in midair, much of the debris will be deflected and not reach the intended target. That is the key--mid air detonation. Merely deflecting the incoming missile may cause it to strike a differrent target with consequences just as disasterous as the intended direct hit would have been. If the missile is destroyed in mid air, then you have shrapnel falling to earth--and it will go where it will go since it is then no longer aerodynamically stable. But, its velocity is reduced somewhat and the chunks are smaller and hopefully the explosive force has been expended. You will have to wait for a while until we get the disintegrator ray developed and on-line.
Report: Palestinian, 18, killed in clashes with IDF in West Bank (Haaretz)
from the article: Iron Dome success excites, but army balks at the bill