: The problem is that I could envision most of Reach's armor sets existing.
: There were a few with thin justifications, like the helmet specifically
: designed for forward observers, but at least only the helmets had an
: obscene number of variations.

Some of the helmets, like ODST, Pilot, or MP, were also designed for other armor systems and are only cross-compatible with MJOLNIR. Same with many of the chest and shoulder pieces and other accessories; there's the ODST chest plate and Marine/Army shoulder pauldrons, knee guards and thigh armor. Many of the ammo bandoliers and pouches available for use were also seen on Army troopers and Marines in the game. This made the customization system feel very grounded and believable and it's something I wish they'd carried over to the subsequent games.

However, 343i's philosophy with their armors has been that all of them are purpose-built for MJOLNIR even if they're derived from other systems. There's three different suits based on ODST alone (Halo 4's ODST, and Halo 5's Nightfall and Helljumper). Not to mention the absurd over-specialization -- there's probably an armor set specially designed for stalking your enemy from behind in low-pressure high-gravity conditions at midnight.

Despite there being a suit for every conceivable mission, though, we strangely never see Spartans wear anything but their one set of armor. Not even their helmets change; say, maybe they might want to don EVA for a mission in a vacuum. This is obviously for character design reasons, but it kind of contradicts most of the variants' reason for existing in the first place.