Actually, I think that prestige retinues are a straight upgrade for
all tribals. (With the possible exception of independent counts that have: no nearby co-religionists, minimal prestige, and no retinues inherited from previous rulers.)
Reasoning:
- The decisions are only available for the primary defender;
- Retinues are available for all tribals;
- Tribals involved in a defensive war tend to call in a lot of secondary tribal participants (eg. vassals, allies, nearby co-religionists)