How are missile defense systems not useful? The most obvious example of a useful missile defense system that comes to mind is the Iron Dome Defense System.
Strategic missile defence systems are destabilizing; game theory argues that if the other side builds such a defence your best bet is to attack before it's completed; failing that you need to build up your armory to a size where it can overwhelm the projected defense. This is why the US and UUSR signed a treaty in '72 specifically banning them (except for one city each; the US didn't even bother to deploy its).
Wasn’t this abrogated?
In the article, the example was a "space-based missile defense system".