Admirable theory but I have to respectfully disagree with it. They are simply enemy types and their different traits distinguish them. They are there to provide different play experiences for the players. Sure we'll see more of a particular enemy type (like the Vex) on a certain planet than another enemy type but they are there to provide variety in gameplay.
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Maybe it could be a compromise. In the levels that MacGrooger outlined, we would see more of certain enemies, though the other enemies wouldn't necessarily be gone, they would just be scarce.
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It wouldn't surprise me if I was wrong, but I just feel it'd change the pace a little bit if it wasn't just one, constant difficulty. Gears of War allowed one person to play on Insane while another was on Casual, meaning the damage done to the Insane player was far greater, so the Casual player would have to look after them. But at the same time, the Casual player would be able to easily kill any enemies because of their increased damage output. It just didn't work as a balanced experience, because the Casual player did all the work. With this idea, everyone does the work they're levelled for.