Even if our missile defense system were 100% effective, that might, ironically, make us LESS safe. Think of it this way: if North Korea thinks our missile defenses are weak, they might make the mistake of actually trying to deliver a nuke by way of ICBMs.... which, even at their best are not terribly reliable (think of all the space rockets that continue to blow up shortly after launch). With crappy North Korean technology, there'd be a good chance any missile they tried to shoot at us would wind up somewhere in the Pacific, or at least nowhere near the target. If, on the other hand, they were sure we have a 100% effective missile defense, then they'd be wise to stop wasting huge resources making missiles, and instead throw that money into more nukes, plus plans for smuggling those nukes to their targets in the US. We have thousands of miles of indefensible coastlines. It would be very easy to move nukes to any location you want in the US. Just rent an apartment downtown in a high-rise, set the nuke up, with lead shielding to prevent detection, and leave it there until it's needed. Then set it off at your leisure, with pin-point accuracy.
@marvin martian