It looks as though the starbase defence modules follow the same progression (5, 10, 20) as they did in the original, v1.0 GalCiv2. Given changes in logistics, defence research cost and the new DA combat system these values are really appropriate anymore.
Through the various patches, defence on ships has become more compact and easier to research, but I haven't seen any corresponding improvement to starbase defence. Similarly, logistics have been expanded, meaning starbases can face larger fleets of small ships with the same defences they had before. While in vanilla GC2 a starbase with say, 35 defence in the appropriate type could at least be fairly resistant to medium-tech small ships, in DA a fleet of small ships with reasonable weapons could wear through those defences fairly quickly.
Given how much construction resources these structures cost they should be defendable. In the vanilla GC2 combat system, you are playing a difficult balancing game, because you didn't want to make starbases immune to small ships. With the logistics and combat changes, I think it's time to revisit these defence values. I think doubling the values for defence would not be out of line.
Also: does a starbase get only three shots (one for each weapon type) or are they broken up by installed modules?