Or more defensive depth
If only we had per-planet turret caps, reduced AIP response rates, rebuildable uniques, and the AI wouldn't just stop after overrunning one planet and sit there on the wormhole for 30 minutes before becoming even more annoying threatfleet, so that defense in depth becomes practical...
For the first three items, there have been plenty of threads about these, so I won't go into this here.
For the last one, yea. There is this obnoxious bug where freed ships sometimes get "stuck" until they get pulled into threatfleet, especially after a planet is lost. They will even sometimes sit there even if there is a totally vulnerable human planet next to it.
I'm pretty sure this isn't some sort of intentional thing to do a semi-contain (trying to prevent you from reclaiming that planet), as the ships in the behavior don't move, even when new enemies come around.
Now if only some of the ships stayed behind (but still moved around on the planet to go after enemies), that would be a cool technique. But right now, too many ships do it, and they stay in place, don't leave the planet when the strategy no longer becomes viable (either too much comes back in, thus outnumbering them, or all adjacent human planets are taken out as well).
This has been reported on mantis several times, and this bug has been around a, to be frank, embarrassing, closely approaching unacceptably long time. (like back in the 4.0 days)