Of course, they have other riders as well. But if it's not part of Froome's job, then why make him the designated "road captain" anyway?
If they had said he was going to be a domestique or a helper, then it would have implied (to those only following cycling during the Tour anyway) that they expected him to do a lot of work in the mountains. This way his role both seem more important, like roundabout says, but people also won't laugh/cry when he loses time every day. Or, a way less likely option, they could simply be messing with the other teams, because they know, he will be the old unbeatable force in July again.