Militant and Progressive Airship Classes

When would I use the Militant and Progressive Airship Classes?

These are there more for a historic reason when the game engine was used for King of Avalon where initally the class depended on the Assault and Guardian points assigned and you could not just change it at will. Now that you can just select the class you would either go for the middle one or one of the two extremes, but not for Militant or Progressive.