Ah man. You guys are putting my in the mood to start a sci-fi nation roleplay now.

For asteroids, even massive ones would have horrible defense (unless you went through the trouble of building an armor shell around it) so could be blown to smithereens long before it reached it's target. Therefore, it's only useful when attacking less advanced races, and if that's the case you can crush them in many other ways anyways. Plus, in my galaxy there's a powerful alliance which bans weapons of mass destruction (before this was in place the whole galaxy was almost destroyed), so if some guys attempt to use asteroids to bomb a planet then they will instantly have swarms of angry ships coming in to wipe them out, and said angry ships can't be defeated so easily.