evacuate almost all the planetary population before the invasion, then retake the planet. It only costs you a few transports, so what, you get a much better combat advantage plus the possibility of gaining free techs.

This is a great idea. Clearly this is a strategy that you'd use if you're behind militarily and in technology but a great idea nevertheless.

As to the OP's questions, I think the answer to all of them would be yes. But I've never built a planetary defense building because I consider them a waste of a tile. I far prefer to be fighting over my opponents planets than trying to defend my own. From my point of view it's not important if your opponent takes a planet or two of yours as long as you're taking more of his than he's taking of yours. As long as this is true you'll win in the end and it's a simple matter of going back and re-taking planets that he's taken.

Yea i know all the dirty tricks to survive against overwhealming odds. When i first started playing galciv2 i did not realise the benefits of CNTRL+N to get myself a nice start with the new tile bonus system.

You never give up.    I've seen various replies from you and reconize your name but none of your posts so far have stuck in my mind. But now when I see one of your posts I'll think "Isn't that the ctrl-n guy?". Anyway, you seem to have a good attitude and I'll probably pay more attention to your posts in the future.

Happy hunting.   

I always build Planetary Defense tiles when I feel I can afford the space.

There certainly are folks that swear by this. I know I had a long conversation with Logic Sequence some time ago about offense vs. defense. He had posted some screenshots that showed taking defense to a high art form. It was very impressive.

This is a legitimate playing style preference, and taken as such there’s no way to argue against it. However, with that said I still truly believe from the point of view of cost effectiveness it’s far more effective to build planets for maximum income, production and research and use those planets to produce as many top line ships as possible.

Your top line ships are capable of protecting your planets as well as taking your opponents planets, and can switch between both roles as necessary. Buildings like Planetary Defense and Orbital Fleet Manager reduce your planets productivity and in the end can’t really protect the planet anyway. That’s why I consider them to be a waste of a perfectly good tile.

I do like when the AI has the orbital fleet manager.

Yes indeed. Besides being able to take out their entire fleet in one attack, it also provides a safe docking station for your fleet to repair hit points for a few turns without running the risk of your ships being taken out one at a time by an opposing fleet.

When I conquer a planet with one of these I'll keep it around until I have no free tiles to build on but then it invariably gets replaced by a stock market, industrial sector, discovery sphere or NLC.