I think the starbase upgrade to assault fighters is broken. When my starbase's get attacked only 3 tiny interceptors launch to defend the starbase, and I have the 2nd upgrade to assault fighters installed (the upgrade you get from researching the last carrier tech).
Looking at the XML, I would be inclined to agree that the module is broken. As defined, the assault fighter module has exactly the same bonus as the interceptor module, so as module upgrades replace the preceding module rather than stacking with it, you don't get anything out of it.
If you wanted to fix this without waiting for a patch, open up StarbaseModuleDefs.XML in a text or XML editor, find the entry that starts with <InternalName>AssaultModule</InternalName>, and do at least one of several things:
- Find the line that says <EffectType>InterceptorFightersCap</EffectType> and change that to <EffectType>AssaultFightersCap</EffectType>
- Find the first set of <Value></Value> tags after the <EffectType>InterceptorFightersCap</EffectType> line and put a larger number in between the <Value> tags
- Copy the section containing <EffectType>InterceptorFightersCap</EffectType> from the <Stats> tag immediately preceding it to the </Stats> tag at the end of the section and change the <EffectType> tag in the new section to AssaultFightersCap or GuardianFightersCap to make the starbase spawn assault or guardian fighters in addition to interceptors.