Edladner: If you have fought through the defending fleet and now have supremacy of the skies, the defenders could not last overly long.
Iraq, Afghanistan, Vietnam, etc. show that superior force of arms and tactical position is inadequate to predict the length or success of a military mission. Regardless, it's a game with aliens about space warfare. What makes sense is kinda secondary. I'm thinking about what makes for interesting strategic and tactical gameplay.
You have at the minimum 2.5 billion soldiers (in gc2 I often used 10 or more transports full of troops, the troops not killed moved on to the next planet) attacking planets with the usual avg of 15 billion inhabitants. In GC3 I have 3 sizes of transport 5, 10 and 20. I move in with fleets using what I need to overwhelm each planet. I also have battlefleets running interference and all my transports are armed.
It sounds as though you've already won the game at that point. I don't see this mechanic having any noticeable effect on this strategy. Your masses of troops would quickly overwhelm the planet's resistance rendering any rescue or reinforcement operation moot. Not that they could do anything since your fleets would destroy them before they could approach the planet.
This is more about a situation where you are roughly at parity with your opponent. (Or the silly problem of planet sniping, where you send in a really fast transports to an undefended colony and take it all in one turn.)
DARCA: No, its all to complicated.
I don't think it's much more complicated than the current system. You drop your troops in, they do their thing. You don't want to wait so long? Send more troops! If the battle's not going your way, pull your remaining troops out and redeploy them somewhere else.
Lucky Jack: The current planet invasion mechanic is simply a place holder. GC2 was much more robust.
I don't know about "robust" but there was at least some kind of mechanic. I'm as interested in seeing what they have planned for planetary invasion as anyone, which is why I'm throwing my two cents up on the board here (instead of jealously hoarding them -- my precious! )