In Bungie's Destiny, we see and hear the lore all around us bit by bit and explore and learn the secrets of our past as the story unfolds. The Grimoire cards do provide that, but individual texts that shed more light on the world would usually go into a codex, which the grimoire replaces. There's also the fact that in a co-operative game like Destiny plans to be, stopping everything to read a short story might not really jive with the pace the game provides. If you're riding solo and want to head back to the tower, sit down on a couch in the bar and read up on the biography of Lord Saladin, or the recent history of the Future War Cult, that could be cool. I'd rather do it in my Grimoire than in a book I got as loot... because that takes up a gear/ equipment slot... and I don't have the vault space for that.