While the best answer is definately "a balanced combination", I have to say that "contested group" is probably the primary driver. While they are seemingly the most vocal, raiders are not the majority of MMO players. And even among those that are raiders, very very few of them are raiders /exclusively/. Single-group content is the most utilized and the "contested" (maybe not the best term) available encounters and zones versus instanced versions of the same are a step above due to simple things like the chatter, sense of an alive world with other people populating it, and a sense of sense of community which is honestly the main prod that keeps people coming back month-to-month. All of the apsects listed are important to the health of any MMO, however without shared content and other people in your gaming environment...why even bother playing an MMO?