it's possible cause the engrams are an important thing in the game and 3/4 of them are commons or rare but unnecessary for a light 32/34 guardian.
maybe they want create an important engram with a sure dropping of exotic or legendary Oryx weapon. More, from TTK The coded engrams will not need more than to be open by the cryptarch, the most hated npc of whole history of the universe (I watch a video of a gamer he said this , but maybe is not right )
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