If you hit Margonite casters like Clerics and Sorcerers (who tend to bunch up together, thankfully), you will hit for the full 100 points and interruption. Their armor is not as strong as compared to the Reapers (Dervishes) and Executioners (Warriors).
Lightbringer's Gaze is incredibly useful in the later stages of the game where you'll be going up against hordes of Abaddon's minions and will need a good, consistent source of damage. Yes, you can only cast it once every 15 seconds, but every little bit counts and you can rest assured that it will hit the Margonites and others for 70-100 dmg each time you use it. Oh, and it's non Arcane Echo-able, since it's a skill, not a spell.