> OTOH, the intercepting missiles cost orders of magnitude more than the offensive rockets, so it might be possible to drain the ammunition supply.
This is the key, I think. As long as you can pound your enemy with cheap hardware forcing him to use up his more expensive one, you're actually successful in your attack - you're bleeding out their economy.
Israel's rocket defense system, the one used to stop the attacks mentioned in the parent post is really good at being frugal in which rockets it shoots down. It doesn't just try to shoot down every rocket coming at it, it plots them and prioritizes ones that will most likely hit valuable or populated targets. It lets the ones that are going to land in empty fields go, since the risk and damage is minimal.
This is the key, I think. As long as you can pound your enemy with cheap hardware forcing him to use up his more expensive one, you're actually successful in your attack - you're bleeding out their economy.