I think the idea is just to find the maximum possible ground defense value, for fun. I think OP managed it - max out stability, get batteries, high command, and star fortress, and install the drone replicator.
As for ground defense not being useful, I think we could stand to see opposition doing more of what the players do. Maybe replace the respawning pre-crisis fleets with intermittent, pre-announced raids, some of which try to tac-bomb (maybe the Hegemony would do this, if hostile), some of which try to raid planets without taking out the station (maybe the pirates would do this), and some of which try to take out the station and then disrupt a targeted industry (maybe the Diktat would do this). This would give ground defense a broader sense of value, and provide a sense of buildup before a crisis.