Roman Full Plate Armor?

[D B Campbell]

... the Legions seemingly didn't so much have a need for such heavy armour- by pointing out that even the armour they did have was full of openings or wasn't super-strong ... L. segmentata, for example, has a nice opening at the base of the neck, and huge spaces under the arms. It must also be borne in mind that body armour is the last line of defense, never meant to be hit.

Aren't we perhaps seeing "style of combat" as a factor here? Your dismounted medieval knight is presumably on foot to fight a single combat, where (as Endre's post graphically illustrates) he requires maximum protection. The legionary lined up in a shield-wall requires less detailed protection, because (in that context, as Matt reminds us) the large body shield is his main protection. (btw I apologise for my ignorance of medieval warfare: I am a dyed-in-the-wool Romanist.)

We do see some Roman cavalry in nearly complete armour, and my impression is that such cavalry get more common as time goes on.

Sean's observation is interesting, because cavalry skirmishing is quite different from infantry battle (to my mind, at least). Also, it is often cavalry that we read about engaging in single combat. And, equally, it is often dismounted cavalry that we find assigned to perilous tasks (e.g. storming a breach). In this context, I have always believed that the close-fitting, head-encasing helmets (the Robinson classification eludes me at the moment; perhaps someone can assist?) are cavalry helmets, and for this very reason.