|The military is becoming ever more reliant upon space based assets. Think for a moment what the consequence would be if those assets were to be somehow neutralized. Given their importance and the fact that they are largely defenseless, to think that they will not come under attack by a potential adversary is naive in the extreme.
Certainly an ABM system can be penetrated by a device designed to do so, but the cost of that device (not to mention the cost of developing it) would be far greater than current delivery devices. An ABM system however can do far more than stop a nuclear delevery vehicule. An ABM system can protect orbital assets.
The largest chunk of the cost of space operations is in the cost of access, the launch costs. In order for an adversary to contest space, to deny the use of one's emplaced space based assets, they will have to launch a great deal of materiel. If an ABM system is emplaced, the cost of such an operation may well be impossible for a potential adversary to bear.