Reading through the allies and scenario rules I'm beginning to think that for competitive games the allies suck really hard unless you are at Battle Brother level. Basically seems like at Battle Brother level allies are just and extension for your Codex with the little draw-back of having to pay for an additional HQ.
If I got the rules correct at less than Battle Brother level the Allies actually start to significantly hinder your game. Even Allies of Convenience are counted as "enemy" units so they not only can't Score, but they actually act as hostile Denial units, denying your own Scoring units if they end up within 3" of an objective... I don't know about you, but I'm pretty sure I'll never run any allies if they are not Battle Brothers.