I wonder if they were trying to avoid the problem of the Echo having no good reason to be able to protect us from being tempered by Zodiark since the Ancients were tempered by him. Throughout the game they say the Echo protects you from tempering. The echo is the remnant of the natural power we had before being sundered. But the ones that summoned Zodiark were tempered despite having the full blown version.

The plot calls for us to fight Zodiark. Well crap how do we stop them from being tempered? Well...uh...IT WAS THE BLESSING OF LIGHT ALL ALONG!