One possible additional benefit to modular ships, though, is war readiness vs. the Combat limit. Say, we build a bunch of combat modules for our frigate or cruiser fleets, but then lock all those modules in a vault somewhere and hand the key to the Council. That way, we could operate a low-C fleet during peacetime, and have the ability to quickly arm it in the event of an emergency (when the C limit is temporarily waived) and then disarm and return the modules to Council safekeeping afterward.
Of course, for this to be practical, the combat modules would need to be attachable within a very short timeframe. If it takes six months or whatever to attach the module, then it won't really work as a short-notice emergency measure.