[quote=Asuras]
So there are defense systems mounted on the space rocks too now? Or are the ships escorting the space rocks?
[/quote]

Defence systems don't need to be.

Point defence weapons aren't going to scratch an asteroid of sufficient size to be used as a projectile. And your enemies heaviest weapons (assuming hard-scifi) will have to spend a lot of time bombarding each rock in turn, or be composed of some sort of warhead to drill into the asteroid to blow it up from within.

Add in that someone conducting a proper bombardment using rocks won't be picking snowballs or dirt rocks but nickel-iron rich asteroids and stopping them is even harder.

Whereas if you just deployed missiles your enemies entire arsenal could easily destroy them before they reached the target world.