I think that they just don't have enough turrets. Specifically, no point-defence weapons in the vein of Subachs. Your average WWII capital ship had a huge number of machine guns and light (ie fighter-scale) weaponry, 20mm guns especially, stuck on the deck wherever they could be fit, to supplement the heavy anti-aircraft guns. FS can't model huge numbers of turrets very well, but methinks that if an Orion had a realistic number of turrets - lots of little Subachs everywhere - they'd be more formidable. I mean, less than 20 guns on a ship 2 kilometers long? Unlikely.