If you're going to bombard something for military purposes, the most effective technique involves using a very large number of nuclear weapons, which is well within the capabilities of anyone who would try to bombard the earth from space.
Back during the Space Race, the idea that the Russians could put something into orbit wasn't just a wound to the national pride, it was a technical demonstration of the fact that they could build ICBM's. If you have the rocket power necessary to lift something into orbit, you have the rocket power necessary to put a warhead on a ballistic trajectory to any point on the earth's surface. So if you can go into space and redirect asteroids, you can obviously launch an ICBM. And at this juncture, it's probably easier to develop the technology for nuclear weapons than the technology to capture asteroids and bombard the earth with them. The only way you could do more damage more cost-effectively with asteroids is if you had an extinction-level asteroid, but there's obviously no strategic value to that.
Bear in mind, I'm only talking about how scary it is, which isn't quite the same as discussing how militarily effective it is. By which I mean--obviously orbital bombardment can be worthwhile, but it would cause no more damage than nuclear weapons, and any magnitude of orbital bombardment that wouldn't trigger a MAD reaction would have to cause strictly less damage.