Actually, I know that the masks were used just in parades, not in battles! But it's something kinda obvious. Think about it: the roman cavalry was more mobile than the oriental cataphracts (who, sometimes, wore masks), so they needed a more wide view of the battlefield; the cataphracts instead only had to charge, retreat and charge again so the mask was some kind of protection to their faces. Moreover some fonts report us that in the hand-to-hand combat they were ineffective, both to their slow movements and to the fact they couldn't follow the enemies' movements, that were too fast for them.
then give them blonde wigs