With the release of enlightment, the rest of the keystone passives seem rather underwhelming.
Currently enlightment offers, a 25%x DMG, 25% regen, 20% attack speed and a +Bonus non-physic dmg of all your dmg types combined.
Thats a lot of bonuses compared to the rest of the keystones.
But if you take for example both of fire keystones, and combine them into a single one, going pure fire remains competitive.
Doing this with cold/lightning keystones, will achieve the same result, as well as simplifying the keystones by reducing the amount from 7 to 4, and making single element more appealing.