If you use a special invasion tactic, like mass drivers, and your invasion fails the target planet takes no damage from the mass drivers. I'd guess it's like this so that you can't just make a cheap transport with 1 million troops on it and smash up all the enemy planets you can't take normally.
However it means even the most massive of failed invasion pretty cheap for the defender, assuming you have better soldiering tech than your opponent and perhaps with the aid of +defence buildings it's not too vital to guard your planets against invasion. Sure you lose some pop, but half the time I'm struggling to get rid of excess pop to improve moral anyway.
I don't know if it'd be worth the effort to impliment; but what if the effect of special invasion tactics in failed invasion was based on the percentage of losses taken by both sides? That is, if you invade a 20 billion pop planet with 1 billion troops and mass drivers just to cause some damage it'd be pretty minimal. Say the attacker loses all his troops and the defender losses half a billion, that's 1.5b out of 21b = ~7% total loss so only 7% of the damage a successful mass driver invasion would be applied. PQ dmg should be rounded down to avoid low scale attrition damage since it can't be repaired. On the other hand, if it's a closely lost invasion you might see 75% of the total troops in the battle killed so 75% of the damage is applied to the planet, so the defender will lose some building and maybe some PQ: a massive battle raging accross the planet will tend to cause some breakage.
This should also provide more incentive to use the special invasion tactics, even if you lose you should cause some damage to the planet. What do you think?