I don't think that the missile shield has ever claimed tobe able to thwart an all out Soviet or Russian attack. What it could do is shoot down a limited "rogue or accidental launch" or a launch by a nation such as North Korea or Iran.
It only takes one nuke to ruin your whole day.
Its a good system in that respect like if Iran was to conceal a missile or missiles in a cargo container or any other ship and managed to launch a salvo with minutes of Washington DC or any viable target along the east coast then yes it would be worth it.
And I hope they try it, give us another Pearl Harbor, actually December 7th is tomorrow....