Putting mass in space as a weapon is just a silly scifi idea disconnected from reality. Even with modern reusable rockets, launch costs are still extremely high, especially if you need enough platforms to hit time sensitive targets. And the platforms wouldn't be survivable. There are cheaper, more effective ways to fulfill the mission.
Not to mention that you get at most as much kinetic energy out as you put in, minus losses to atmospheric drag and what's needed to drop the orbit (you have to burn retrograde to return from orbit).
