Flying machines can be prevalent in a land based game of strategy and warfare, and they also aren't game changers. A Dwarf in a gyrocopter can be killed by a volley of arrows, or a cannon, or a catapult, or yes indeed, a Griffon rider. Ships however, need water, and no one but you two will be using them. Which was the point. Clock began this game with a specific set of standards in mind, and people deviating from that, no matter how pointless, can change how people view the game, whether or not they need to add anything to their sheet.