[PTTG]

Just rough estimates here, one 100-ton rock that could hit the Earth at dangerous velocities, if it instead hit the moon, would probably have much less than 50% efficiency at dropping moon rocks on Earth at dangerous velocities.

That's not to say that it's a bad approach; a lunar debris storm after a, say, Ceres-sized object strikes the moon would result in a very interesting world exactly because it is one with a heavy rain of smaller impacts. Plus the moon would be glowing orange, so that would be cool too.