Yes, the Unit takes damage event fires before the actual damage is applied. However unless your Earth Shield ability is selfcast only (compared to the one in WoW which can be cast on any friendly target), adding unit to the "Unit takes damage" event is inefficient.
More than that, a damage detection system allows you to catch when unit takes damage and modify that damage. It's also set up so there's only 1 damage dealt, not that there's one damage dealt by unit and then one modified by your spell.
It will also solve things like unit with full health taking damage and your trigger healing that unit before the damage is dealt, thus the heal is negated --> with DDS if unit with full health takes damage, the heal will be applied correctly.