I second Master Belisarius' opinion about the AI having little strategic concept about attack locations and also that they leave captured planets behind undefended.
It is also true that in the example MB used the AI tried to capture a well defended world, however, it could not have known how well defended it actually was. The world had two defense stations (which could have been space yards as well), five weapon platforms and a few missile sats. If the space stations were scrapped (Okay, a little cheating, but which non-homeworld has two defense bases anyway?
), the attack on the planet was well executed by the AI (troops stay behind; WPs are destroyed; troops move in for the capture, while sats are destroyed). So that's not bad.
A few turns later in the same testgame another AI fleet showed up that was strong enough to capture the planet even with all defenses. I reinforced the defenses of that planet with a few ships to see what would happen. The AI was now attacking a different planet that had no defenseive fleet and captured that instead. So the AI does seem to attack the least defended planet in a system.
IMHO the AI handles planet capture not too bad. The thing that really hurts is that they leave the planets undefended.
So why bother with it, some might ask? Because it's fun
.
Rollo