Taking potshots at the missile shield. Everyone knows that. It's like hitting a bullet with a bullet. Just detecting them would be challenge enough, given enemy anti-radar technologies. Tests conducted to date have been abject failures.
Any fool can see that.
In other words, it would work too well. So determined are NMD's many critics to shoot down the idea that they do not seem to have noticed that their two main lines of argument are diametrically opposed to one another. Either NMD is an obvious technological fantasy, which no sane person takes seriously. It can't be both. In fact it is neither.