It's nothing but a buggy creature stage and all missions are practically the same.
Not really, if you have "download my buddies creations only" off or you download some missions every once in a while you have a good variety.
I was imagining planet capture like how you said though, not just destroying their cities.
Whenever I encounter a Warrior empire or whatever, they always try to make me pay for protection so I just run in and planet bust their asses to the stone age.