Why does everyone think that it's either:
"No Orbital Bombardment"
or
"Orbital Bombardment which is capable of leveling civilizations without thought"
???
I mean, as awesome as our fleets are, they have precisions weapon designed specifically to hit a small moving object very very far away (space is big) ... then to do damage in such a concentrated fasion that it is capable of penetrating the thickest armor, strongest shields and densest PD systems... these weapons aren't the same thing I think of when I think "mass destruction from orbit"
I honstly feel that we can have a balance with Orbittal Bombardment which allows for the option while precluding it as a large scale replacement for invasions ... and still fitting in with our diverse views of reality.
In realistic terms (as best as possible)
Yes:
-A ship in orbit can CAN fire on a planet
-These weapons CAN cause massive damage to the population / infrastructure in general
But realise that:
-On a world with a population of only 5 billion (like ours) there is a HUGE infrastructure. There are more cities than can be counted, and within them more city blocks, buildings and people that your (relatively) puny dozen or so laser Mk3s must select from as targets. Honestly, a week may be a long time, but there is simply too much for your precision weapons to fire at to make much of an impact.
Now imagine a world of 27 billion! Maybe a PQ of 16-18? Imagine the huge stretches of infrastructure that make up a SINGLE manufacturing sector. It would take a lot of fire power to reduce such an area to nothing. Damage? sure, destroy? hmmm....
Granted, later in the game you will have hull sizes, fleet sizes and weapons of the right scale to turn a planet into ash, but I would imagine planetary defenses will rise to meet the challenge. Just as we no longer build buildings from wood and stone, in the future of GalCiv2, I would expect concrete and steel to phase out as well...
...not to mention the massive planetary shields and such which I can only imagine are a part of a "normal" planets repitoire of defenses.
Personally, the weapons I envision doing the most damage to planets, would not be the same weapons I'd fire at ships (large area effect vs. concentrated damage ... IE stick of dynamyte vs. a shaped charge)
Balance, that's the point.
-Let people fire on civilion populations (Hell, I'd love to myself

)
-But don't give attackers an unrealistic (or unbalancing) advantage over transports.
-Early game weapons would likely be ineffective, at most killing a few people and reducing the social/military/tech production by a smal fraction.
-Late game weapons capable of demolishing buildings, eliminating tiles (-PQ) and killing people by the millions would be offset by planetary defenses geared to countering/lessening them. (I can smell two new tech trees... perhaps alignment based!?

)
mmm... nukes, massive napalm-esque payloads, viral payloads, orbittally launched core detonators (oh, evil races have it soooo good!)
...
First Time I've chimed in on it, and based on the frequency of this topic, likely not the last