Just because Destiny is primarily an online multiplayer game doesn't mean it can't include a compelling narrative.
Bungie.net community manager DeeJ has been answering pressing questions concerning the team's anticipated new MMOFPS. Firstly, he confirmed that your character will be "totally customizable, and you'll evolve and change over time as your adventure makes you more dangerous." Sounds a little like Fable .
Furthermore, there's going to be quite the "rich narrative" involved. While you will spend the majority of your time gunning down baddies with your friends, you can also seek out pieces of the story as you progress. You can even strike out on your own to explore and conquer, although that won't always be advisable:
"I can tell you that Destiny will revolve around a rich narrative that is deeper than any story we’ve ever told. While this world will be populated by other players, there is nothing stopping you from charging through and crafting your own adventure unaided by other gamers – although you may find yourself wishing for an ally or two when the Fallen drop a Devil Walker packed with sweet loot in your path."
Personally, I'm not a fan of such storytelling and I'm relatively certain that nobody is going to be picking up Destiny for the plot and character development. It's almost entirely gameplay-oriented, obviously. But you can still have a few interesting things happening on the side, which makes for a more involving experience.
Related Game(s): Destiny