Missile defense systems are the modern day equivalent of the Maginot Line. Everyone behind them will feel good until they are defeated by an enemy with an ounce of creativity.
The Maginot line was very effective. So effective in fact, that the Germans refused to attack it, and instead went around it. Stupid French, not building it all the way to the coast.
The effectiveness of the system is only as good as the ones in charge of it. If they build it, then sit on their laurels and think "we are safe now" then you are correct, it will be a failure. If on the other hand, it gets built and considered part of an ever changing layered defense system, then it can be effective.