As it is now, the AI and any human player won't really bother building a military starbase on larger maps. The offensive&defensive bonus only apply to the area of effect generated by that base, so outside of small map or if you're really equal in strength to the AI (so that might apply to the highest difficulty level), they serve basically no purpose.
So, what if, only military starbase would extend the range of your military vessels? As it is right now, just about any starbase extend your range. That prompts the AI to over-extend itself in the hopes of reaching you (btw, something has to be done about that tendancy to declare war to players they can't reach, it just weakens the AI, shifiting their priorities to out of range aliens while they're pounded by their neighbours), or the next resource. Maybe we could make it so that civilian vessels like cargos aren't affected that way and can still use any starbase to extend their range?
Also, maybe add a module to military starbases to repair ships faster when they're nearby?
What do you think?