Interesting observation, Guy.
The only Dark Ages I game is using SAGA where small groups of armoured warriors, representing a chieftain’s body guard, stand out from larger groups of the unarmoured.
I also have 3 Chariot Wars (Bronze Age) armies with large numbers of unarmoured (not counting stiffened linen, thick cloaks etc) infantry. I often stick an armoured figure on a command stand for these units to represent a decidedly more affluent leader. Several of these units don’t even have shields.
All three armies (NKE, Hittite, Mycenaean) do have small exclusively armoured infantry units that represent mercenaries, Royal Guards & warrior classes.
The point of my digression is that I think in armies where armour was not de rigeur (eg the Romans), the rules should be to allow some grouping of armoured fighters who would have reasons to be an homogenised unit, the odd armoured warrior amongst the “naked” masses and not worry too much about the whole matter as we already make so many compromises in turning historical reality into a wargame.