Oftentimes methods like tidal distruption, mass drivers, core
detonation, etc are used from orbit before the attack commences. Those
are supposed to have devastating effects on the planet, destroying
almost all the improvements, damaging its quality, and reducing the
advantage of the defenders by alot.
However
those things ONLY take effect if you CONQUER the planet. If you blast
the planet with asteroids massive damage is taken if you win, but none
if you loose?
A much more reasonable approach would be to
seperate the ground attack and orbital strikes. The orbital strikes
would just kill people and damage the planet, and the ground attack
would be a seperate ordeal that MIGHT Come later.
Even if this is not implemented though, the damage to the planet should occur regardless of victory status.