We need one mage to decurse and one mage to get adds. Rangers could get adds, but not nearly as well as mages can due to the ability to curve spells around the boss, and on moving targets.
Now if rangers had a decurse arrow that would open up group composition a little. But for the most challenging shards, we’d still prefer to bring mages for AOE, Mana Shields, and etc.
And another thing, rangers are expensive! They need both Intellect and Strength potions, whereas mages only need Int (and Empowered Strikesof course).