[TheDemon]

While it's true nations with DV benefit, nations that can summon Demons and Undead benefit the most, since they don't have to rely on gold income. AND those nations are already very powerful, since demons and undead are already very powerful. Yomi and Argatha and other nations with troops with Darkvision can benefit from casting Darkness in the battles, but thye need gold to recruit troops, which utterdark takes away.