On the surface, this does not seem like a particularly major issue. However, the lack of the structure namespace causes Minecraft to attempt to deserialize the custom structure data as if it were native to Minecraft. This normally would not be much of a problem, but if HotM is removed from the game and a world that had been generated with HotM is loaded, then Minecraft will be unable to save the chunks with the, now missing, HotM structures in them, effectively corrupting the world.