These chaps play a bit funny.
You need to have them to get Legates. However, it makes no sense to put them in the legionary units, as they convey no attack benefits and if a unit is lost you would lose an additional army break point. Consequently we will see them leading their brave boys from behind, or hanging around near skirmishers to try to rally them when they break.
It's just not how I envisage centurions behaving - or have I missed something.