Personally, I think that, regardless of its power level, Blade of Yoth is a very good card for Arkham because of the thematic point it makes, that the Mythos is not inherently evil. The point of Lovecraft's creations wasn't that they were hostile to humanity, it was that they were largely indifferent to them, and the cycle that gave us this card's name is one of the best examples of that: in the Forgotten Age, neither the Valusians nor the Yithians were, strictly speaking, evil, they were only trying to save their homes. Yig, the Father of Serpents, only fights you if you threaten his children, and in one resolution is willing to give you gifts for helping his children.
The Mythos is not a monolith of evil; it's a collection of non-human individuals, oftentimes possessing as much capacity for good and evil as the investigators. We can't sing Kumbaya with them, our needs are too diametrically opposed (again, the Forgotten Age shows it really well with Ichtaca's point about Earth belonging to humanity now, not the Valusians), but that doesn't mean that we're inveterate foes, utterly incapable of benevolent interaction. We're people, with all the messiness and conflict that brings with it.