Although I will agree we need to keep up with China/Russia...couldn't Earth targets be pummeled by existing satellite technology @Kamagi?
I'd have to look up how far the moon is from other planets, that might be hospital to humans to make an accurate assessment of how cost worthy it would be.
We could drop stuff from orbit, but we'd have to put it there first. The "Rods from God" concept (SDI) would have used high density rods traveling at high speeds, and causes impacts with the force of nuclear weapons.
The problem, however, involved getting objects (likely Tungsten rods) into orbit in the first place. While a lot cheaper (here, on earth), those rods with a density close to that of gold, would be worth their weight in it by the time they got up there in orbit (shipping and handling would not be cheap).
On the moon, though, you could mine the bits you threw at earth, even launch sizeable rocks...
If you watched
The Expanse, Marco Ineros (sp?) (rogue ''Belter') was launching stealth coated rocks at earth in an attack, with devastating results.