On the point of only ever building 1~3 templates for a ship type, I feel that lack of usefulness of modularity, in AI War Classic, is partly a product of overall game variety.
- There's two optimums in conventional weapons: either combined arms, or max one tech, even if the AI sends only one type of ship at a time.
- Players can never have enough infinite-range weapons and plasma sieges, so that's how your Spire Fleet generally turns out.
- Shields are the only defensive options for Spire Fleets, Champions and Modular Forts. They don't have access to other mechanics like health regen, armor, phasing, teleport, etc.
If that's how the combat philosophy is maintained, then yeah modular isn't useful. You'd want the viable paths offered in Gratuitous Space Battles to justify asking players to kit out their own ships. (Multiple viable options in all categories: defense, offense, mobility, utility).