I think the reason they changed the shoulders, is for the generator build. The build was a poison build but it use the singularity rune to dump essence so that you could generate as much as damage from Leger’s Disdain. With the shoulders you can play poison mage and not have to use a skill that is not poison.
Also every single mage build besides Trag’Oul Blood Mage uses Singularity anyway so now you get to pick either archers or poison while still getting singularity.