The problem with Companions and Cataphracts is the silly speed model used in the game.

In MTW cataphracts were formiddable but they were also very slow and tired quickly (as cataphracts should), the Byzantines even had an alternative cavalry unit the Pronoiai Allagion (sp?) that whilst weaker were as fast as normal knights to make up for this.

In RTW Cataphracts are just as fast as Companions, have similar attack stats and craploads of extra armour so that they're much better in a melee.

With this in mind then Companions become redundant.

Cataphracts should be much slower and should tire out faster than other cavalry units, the Companions would then be the cavalry that one uses to ride around the enemies flank and the Cataphracts what one would use to just slam headfirst into an enemy line at the point of crisis. Both units would have distinct uses.