My case was about not having radar at all. Having GPS could buy you some time and start braking already based on GPS even if the radar is still out. Yes, might burn some additional fuel but burning too late sucks harder I suppose.

Also: even tough I couldn't find anything about the navigation (or rather localization?) accuracy of the Moonlight system, I'd expect it to be better than 100m, but I have nothing to confirm or deny this.