Orbital Bombardment-Regardless of if it’s with k-rods (kinetic energy weapons) or directed energy weapons, orbital bombardment is a threat to any planet that doesn’t have city shields.  Since a shield that can protect a city isn’t something that can be moved easily, the rest of the planet is vulnerable.  However, area bombardments are not used by any of the major races until the Horizon War.  There is a moral component (there hasn’t been a war that has involved reducing a planet to glass before then), a logistical component (if Changelings killed off their food source, they’d starve), and a political component (for the most part, the usual result of somebody taking over the high orbitals was a quick surrender, followed by political negotiations).

During and after the Horizon War, mass bombardment of planets became a valid strategy, especially with Changeling-held worlds.  The capability of such bombardments made grav-armor more important, as such vehicles were hard to target with orbital bombardments short of a saturation strike.