[Alpha Kodiak]

I too have seen the AI use planet destroyers against enemy colonies, and yes, the big problem is that they don't escort them. It can be effective, though if the target world is not protected by ships, fighters or drones, and the strategy on the planet destroyer is "Don't get hurt". As near as I could observe, the planet destroyer enters the sector with the planet and combat ensues. The planet destroyer runs for the corner, and the planets defenses are unable to do anything to it. After combat ends, the planet destroyer can destroy the planet.

While it is probably mostly a waste, it does give the AI yet another twist, and it can be a surprise to even a human player if they are not paying close attention to their entire empire.