The way the AI is now makes it unpredictable. That may be the intended effect. You never know if a fleet passing through is going to warp onwards, return home, or attack. So far as defenses go, the AI defends with sats, WP, and bases - not fleets. I recall some very tough bases to crack made by the Cue Cappa and Phong. I also recall some Terran and Xichung planets that had far too many seeker WP for comfort...