The AI could have beaten me to two high quality planets , but their colony ships took the scenic route and I nabbed them both. (Only by one tile, but that's as good as a mile.)

Were you fighting... the Minbari?

I play normal, and the airport invades my planets. Go figure.

On the one hand, I think it's realistic that the AI will sometimes kill everything, sometimes forget to kill everything (Have you ever played a multiplayer game where you've stuffed something up in a situation you've been in before and thought "Dang! Last time I did the right thing!" and had to deal with the error you made? Same thing.) On the other, it's pointless leaving those things alive because the race that's been attacked can't A: Build and populate a Transport or Invade ship to populate/invade a planet or B: Build enough of a war fleet to get back into the game because there's no planet to sponsor the surviving shipard or populate the Transport/Invade ship.

On balance: The AI should be told to Kill. Every. Damn. Thing. Leave realistic stupid AI decisions to something less obvious: not growing populations so well, etc etc.

How about an AI fleet defending a planet, pop out destroy my force that I am building to attack planet.  Nope, waits for to bring enough ships to destroy defense fleet.

I guess it'd be a good idea. Often it doesn't even attack my ships which are far weaker than his fleet passing by. 

I remember once I played as Terrans. All enemies were set to normal difficulty with one exception - the Dreghis who were set to genius or incredible. Surely they went to war with everyone and started swallowing the galaxy like the Golden Horde, but they sat at the opposite corner so I could build up.

At some point Altarians surrendered their resources to me. They were next to dreghis, but it'd take 20-30 rounds at least for me to start bringing first ships there. So I used a starbase which at that point still provided some protection. I rushed a shipyard, build a ship with legions and stationed it at that starbase. I started building military ships.

At the worst moment I had just two planets in this corner and I needed to recapture them every few rounds with this legion ship and a few battle ships I built.

What's the problem: while passing by they never touched the shipyard.