Would Aliens use rocks as a bombardment method, consider the following: (not mine, all credit to whoever wrote the original)
Rocks are NOT ‘free’, citizen.
Firstly, you must manoeuvre the space vessel within the asteroid belt, almost assuredly sustaining damage to the ship’s paint from micrometeoroids, while expending the fuel.
Then the rock in question must be inspected to ascertain its condition. Should it pass muster, the ship must prepare the potential ordinance for movement. Finally, the ship may begin manoeuvring the warship to abut the asteroid at the prepared face (expending yet more fuel), and then begin boosting the stone towards the offensive planet.
After a few days of expending a prodigious amount of fuel to accelerate the asteroid into an orbit to hit the planet, the ship may then return to the planet via superluminous warp travel and await the arrival of the stone, still many weeks (or months) away.
After twiddling away the time and consuming food in the wasteful pursuit of making sure that the planet does not launch a deflection mission, they may finally watch the ordinance impact the planet (assuming that the ship does not need to attempt any last-minute course correction upon the rock, using yet more of the fuel).
Given a typical (class Bravo-CVII) system, we have the following:
Two months, O&M, Titan class warship: 5.2 Million Credits
Two months, rations, crew of same: 1.2 MC
Paint, Titan class warship: 2.5 MC
Dihydrogen peroxide fuel: 0.9 MC
Total: 9.8 MC
Contrasted with the following:
5 warheads, “Planetary Destruction Grade”: 2.5 MC
One day, O&M, Titan class warship: 0.3 MC
One day, rations, crew of same: 0.0 MC
Dihydrogen peroxide fuel: 0.1 MC
Total: 2.9 MI
Given the same result with under one third of the cost, will have saved a massive amount of money and almost a full month of time, during which the warship may be bombarding an entirely different planet.
Purser Level XI,
Office of Interplanetary War Costings.