So only GLD/PLD and early CNJ/WHM THM/BLM only really use them so I just kinda thought

MNK/NIN so basically if you equip their weapon both slots get filled. However you can glamour their off hand slot to mix and match different weapons? Kinda like how NIN Thancred used a piece of sharpened stone as his secondary dagger. So a MNK could have a claw in one hand and an emperor's fist in the other.

BRD could be the Harp they use. The BRD teacher uses the relic Harp as his. So why can't we?

MCH the big box on their hip. Perfect way to add in an emperor's MCH off hand. Maybe even some other ones? Like an Allagan one, Moogle one, etc

All cosmetic though, so no need to balance stats for them all.