They're a confusing signal to drafters. They go into only one faction, so they are less flexible than enemy color cards in this particular set. They are much worse than actual three color cards at signaling the fact that this set is about three color factions. They eat up card slots that we then can't use for either the more flexible enemy color cards, or the more identity-shaping three color cards. Allied color cards in this set would bother me a great deal aesthetically. It's not that we
can't do allied color cards, it's that I don't see why we
need them. I do see actual downsides to adding them to our set, however. If we don't need allied color cards to get across our theme of wedge factions, why do so?
And then there's the fact that in your pitch for this set, you literally said: "Game play wise, I'm imagining a wedge set, where every one of the 5 factions actually is focused on the enemy color, so unlinke Khans did it." How is adding allied color cards focusing on the enemy color? Even in Khans, where it would thematically have made sense for, say, the Abzan faction to have a
uncommon and a
one because Abzan was "centered" in white, Wizards declined to do that and instead used two enemy color cards per faction because that makes for a more flexible draft environment.