Its a good idea, but that would be alot of work on Bungie's end to remake legendary versions of every blue piece of gear. Perhaps instead, Bungie could make it so Etheric Light can be traded with The Speaker for motes or with Xur for strange coins. Maybe 5-10 per Etheric Light. Maybe they could be traded with vendors (Other than Banshee and the Cryptarch) in the tower for reputation. Or, they can be used to infuse a piece of legendary or exotic gear. An item can only be infused once, and will give that gear a +10 in attack/defence regardless of its current value. This will be limited by the level cap of course.