I'm sure many of you are familiar with the theory of comparative advantage from an introductory economics class, or with the various theories associated with cell specialization from Biology. For those of you who aren't, these theories can be summarized fairly quickly; in a large society of humans or cells, the society produces the most if all the individual pieces specialize as greatly as possible. It does not behoove the human body as a whole, for example, to have brain cells trying to perform the functions of a kidney as well as a brain, or vice versa; it is best if all the cells concentrate on doing their particular job as best they can. This general theory has been true for any large society humans have studied in which the individual components have their own strengths and weaknesses; the concept can be generalized to say that a society of specialists simply gets more done than a society of well rounded, self sufficient entities.
And I think this general understanding of societies composed of individual components each with unique strengths and weaknesses (be those components cells, people, countries, or otherwise) can be applied to the relatively simple society inside World of Warcraft: a large group functions best if all of the pieces inside play to individual strengths and admit to individual weaknesses that other people/classes in the community can shore up and perform themselves. This theory would recommend Arcane/Fire/damage oriented Mages, Assassination/Subtlety oriented Rogues, Marksmanship Hunters, Holy/Discipline Priests, and so on, and I think this is precisely what many of us are finding is optimal in the game.
Keep in mind that this theory does not and cannot work for solo or very small group play; the concept of comparative advantage only takes effect in larger societies. When alone, well roundedness is powerful and important, as a player is forced to be his own healer, tank, and damage dealer, all in one. But as the society involved gets larger and larger, I see the fundamental rules of Comparative Advantage becoming increasingly valid.