Reenactment Legion Uniformity vs Variety

[Salvianus]

Here the question was:

"Why would there be soldiers that have no metal body armour?"

The first logical step would be to look at literary and pictorial sources, and to try to find an answer.

Of course, for the Late period, there is Vegetius (I.20), specifically contrasting the early Army with that after Gratian (367-383 A.D.):

"Circumstances dictate that we attempt to recount the types of weapons with which recruits were either armed or protected. But the ancient customs have been completely abandoned, for, although the example of the Goths, Alans, and Huns has been advantageous for cavalry weapons, infantry continued to be undefended.

For, from the foundation of the City to the time of the late Gratianus, infantry were equipped with both cuirasses and helmets. But when field training was ended through negligence and laxity, the equipment - which the soldiers seldom put on - began to be seen as heavy. Therefore they first requested the emperor to stop issuing cuirasses, then helmets.

So, fighting with unprotected bodies and heads against the Goths, our soldiers were often destroyed by large numbers of archers. And even after so many disasters, when destruction came to many cities, none of these troubles led to the restoration of either cuirasses or helmets. So it is, those who are unprotected in battle, exposed to wounds, do not think of fighting, but of flight. For what should the infantry archer do, without cuirass or helmet, unable to hold a shield at the same time as a bow?"

However, most authors I recall recommend taking Vegetius' handwringing with a pinch of salt, e.g. as perhaps referring to equipment shortages after Adrianople. It certainly doesn't settle the matter.