The idea isn't to bad, I would hope that when you join a faction that it becomes permanent. Kinda like the Horde/Alliance in World of Warcraft and if you do leave a faction you lose all reputation and items/gear earned? Then once a month Bungie have a faction War on the last weekend, were its a 3 team death match. All wins and losses are recorded to each faction and the overall winners get a unique item (doesn't have to be exotic, a weapon or armour). I think it would make it a bit more competitive but not in an aggressive way which can be enjoyable.