Very useful. Hypersonics shorten the window for defensive engagement but don't completely eliminate it. For example in surface naval warfare anti missile defense is layers of counter systems / munitions. The innermost layers of this "onion" of protection are fast reaction systems that function essentially at line of sight (SeaRAM and similar missiles, CIWS). That an incoming hypersonic is trying to shorten the detection window is all the more reason that detecting one quickly after it comes above the horizon is valuable.
