On rare occasions when playing on Expert level, the AI will attack via port. It's usually the Shimazu and/or Mori, attacking Owari in midgame as their fronts approach central Japan. I've noticed they tend to do this when they do not yet occupy a central province with a port. Kawachi is also often a target if a port has been built there.

In one very strange game the Mori kept attacking Owari via a port jump round after round. They would attack with one unit, and lose every time. I used the province as a taisho (general) training ground and ended up with 4 6-star generals in the same game.

The port jump is a great way to raze koku (pun intended), severely hamper enemy troop dissemination and reduce their income (by destroying ports), and prevent significant infrastructure development. It diverts attention away from their front line and generally stymies their focused march to the capital and beyond.

Contrary to the opinions of many who post here, I feel the port jump is a legimitate tactic. In some games it is the only way to keep the Shimazu or Mori in the west and the Uesugi or Hojo in the east from building up those massive stacks of armies after they eliminate the local opposition. As Japan is a much smaller area than the area covered in MTW, not having to manage ships is not so unrealistic to me. I also tend only to attack those that attack me, just to make the game more interesting. Once they do, I wage open war with them, and use port attacks as I deem necessary.

One of my favorite ways to attack via a port is with an army made up entirely of battlefield ninja. The enemy usually responds with a small army because the attacking army is small. You have to fight these battles on the field usually, but enemy troops almost always run when they see an army of BFN headed their way. Well, actually they only see the taisho unit--all other units are invisible until they attack. It doesn't take long for your BFN general to attain 4-star rank, and by then his ninja troops have also gained honor. Before you know it you have a very powerful, albeit small army of elite BFN, capable of defeating armies 4-5 times its size. These armies make excellent assault teams againt castles that are not heavily garrisoned, and are also useful in bridge assaults as the enemy cannot see them sneak across the bridge.