I found the ERE rebels to be a nutritious source of family members. I precipitated their appearence during some slash-and-burn forays into the Sassanid hinterland and they appeared with a truly massive stack of peasants and about five family members. Over the course of the next few turns, three of the family-members switched over to my side - and one of them was suprisingly good (positive, good commander, very loyal (ha!), and lively, and some others).

So rebels aren't all bad, provided the pop up somewhere predictable.