6 was set aside for later use. I think for an Early Machaka? It just never got used and now trying to turn it on gets an error if its not done carefully.
23, 24, 25 arent actually unused.
23 and 24 are the "Special Monsters" AI's and are the "nation" controlling some random events and random summons
25 is the AI of the Independent's
You can actually turn them on in a game, give them a capital, sites, a god. I do that in my Chaos games. SURPRISE!
http://www.dom3minions.com/HostedGames.htm