Or since a DPS' Damage Shield is meant to help them maintain their rotation, like Black Mage's Manaward for instance, Shade Shift can be more universal to help ensure it since we have it, thus allowing us to have a different Role Action slotted instead. Yes this isn't entirely about a rotation but it is for Black Mage.
Another reason to use Shade Shift over Arm's Length there is the fact Arm's Length knockback protection only lasts 5s whereas Shade Shift lasts 20s. Not too big a deal I know but still![]()