Until the late game in which you come under heavy attack repeatedly at certain key worlds, building Defensive bases is absolutely and totally a waste of money. The AI's target selection AI has this habit of getting into a rut: I've noticed certain worlds, despite being massively defended, will be subjected to assault OVER AND OVER, while other worlds are completely and utterly ignored. I've seen the AI show up with what had to be hundreds of ships to assault a Gargantuan Space Port: A 5000-size spaceport with over 80K shields and 30K firepower. They got their asses handed to them in seconds, in part because two of the attacking forces were also hostile to each other and began exchanging gunfire with not only the space port, but each other. The spaceport failed to sustain any damage at all. This did not stop them from trying it again.
There is still absolutely no reason to design a defensive base at this point... make a "mega port" with ridiculous amount of weapons and shields and armor etc on that planet... Ports have the advantage of sitting ON the planet while defensive basis are off to the side and often can't even reach the action.
Plus, you can use things like the devastator pulse on such a port... but frankly, those suck ass. I prefer more torpedoes to devastator pulse.
I do not have a superman complex; for I am God, not Superman.