The Halo novels are a great show of how to really deeply grow an EU, and I hope they make Destiny books, but the game does have a full story. The Harry Potter comparison is that Destiny leaves out some of the small details in the game like the social structure of the Fallen because it would make the game drag on and be annoying to those that don't care. The cards fill those places in the way the Harry Potter books have all the details that the movies left out. I don't know which Halo books you're talking about but I'm going to assume the Traviss books because those were mistakes and an example of a writer that doesn't respect a series of great books.