Destiny's power system has always been vague, but I would say there's always been a few concrete rules that have remained consistent over the years, one of them (in my opinion) being that in order to wield the power of the light, it must be "given" to you via a ghost. This rule was completely shattered and has been laying in pieces for years now, ever since Ghaul was able to harness the light without a ghost. Ghaul broke one of the central rules of the universe, did what no man has ever done before, and made himself a lightbearer. And since then its never really been brought up since. Not even during The Witch Queen when more rules about the universe were brought into question. In hindsight, I think it should have been a bigger deal.