It boggles my mind that you have so little respect for my opinions that you do not even do me the courtesy of accepting that I truly believe what I am saying. I guess we aren't on the same level of mutual respect thatI had thought we were.
Halo is a game about scale, and it always has been. The skyboxes, especially on the Halo rigs, do a good job conveying that very broad scale, but there needs to be an intermediate big, somewhere between the level geometry and the skybox. That role is filled by what are primarily low poly, low res objects of great size and great distance. The result is not only a more convincing realization of the world, but also something which can give depth to the game, where people can continue to be impressed long after the first run of the game. While that continuing sense of awe doesn't seem to be important to you, it is certainly very important to me, as it is one more thing that sets Halo apart from other games.
Your argument of these touches reducing memory that can be used on other areas of the game doesn't hold very much weight either. When we are talking about games that have eight bajillion polygons for the AR, a few hundred or even a few thousand polygons in the skybox are just a drop in the ocean. Reclaiming those system resouces wouldn't allow for dramatic improvements in any of the areas you mentioned. Wallpaper backgrounds won't make enemies noticably more intelligent, they won't give back enough to suddenly allow x16 AA, they won't get the game running at sixty frames per second. Ultimately, it is a conscientious decision in which a very little goes much further than it could if devoted to another area of the tech. All of those other alternatives I mentioned are the result of sacrificing more resource heavy elements and clever coding.