30 can stop 2000? At 30 firing every 6 seconds, that would take 400 seconds to hit each one once. Do those attacks actually take 6-7 minutes to get through your planet?
I don't mean to suggest they're alone when they're doing that. Essentially they allow the tractors to cycle and maintain a grip on things. 6-7 minutes seems longer than that was taking in my experience, though. Hmm, maybe I'm misremembering the spider turret count there. I built and rebuilt that system so many times I may have had 50 or 60 there by the end of the game instead of 30.
The main thing I can tell you is that, several times in my most recent game, a system with a respectable number of automated defenses and a few hundred ships held against enemy forces in the 2000-4000 range in large part because by the time my main fleet got there, none of the enemy ships could move anymore and were therefore incapable of threatening any other systems. Perhaps what they're best at is covering mistakes, which may be why Suzera has no idea what I'm talking about.