And the ability to invest further cash into the game. Bungie themselves have compare the base game to being only the opening chapters of a 10 book series. The plan is to release a new game every 2 years, with the evolution of the "books" being expended over this time. In between releases there will be sizable DLC (and you can assume sizable == number of and cost), with each DLC expanding both the storyline and the playable universe. If you can afford all this you can afford XBL, no brainer.