It's important to note that in a single player game rebuilding Harrenhal can be a good idea since it seems the AI never uses dragons in battle/sieges (either it's unable to or it just chooses not to), but any large dragon deployed in a siege against a rebuilt Harrenhal (or at least one that wasn't burned in the first place) can result in the entire county being reduced to rubble again. So if you rebuild Harrenhal in single player, hatch a dragon, and have someone successfully siege the castle, don't use your dragons in the siege. If you're playing multiplayer and another human player has a dragon, rebuilding Harrenhal is more risky unless you know they aren't going to attack you and know not to deploy the dragon to retake a captured Harrenhal.