Over the course of the years and years since classic (seven of them, to be exact) we've seen hybrid classes rise to ascendancy. The way hybrids were balanced for pure DPS changed to be much closer to pure DPS classes, and since all healers and tanks are hybrids there's been competition between each for both of these roles (considering that the two new classes added during WoW's life, death knights and monks, are both tanks and monks are also healers, competition has been necessary) making hybrids more attractive. However, it was really the addition of dual spec that made hybrids start to live up to the ideal of the hybrid class - with dual spec specialization, a druid can choose to have a tanking and healing, or tanking and ranged DPS, or healing and melee DPS specialization ready to be selected at the touch of a button.
However, it's never as easy as all that. Yes, a paladin can have a ret and protection spec, or protection and holy, or holy and ret ready to go. But he or she still needs to gear said spec. If you intend to heal for you raiding, tank for five mans and flex, and go ret for fun you'd actually not only need to hit a trainer from time to time to drop a spec, you'd also need three sets of gear ready to go. And it is this very limitation, so woven into the fabric of the game over the past few years that I myself have almost entirely forgotten about it, that is about to be bent further than it ever has been.
Make no mistake - Warlords of Draenor will change not only what stats we want on gear, but how we use that gear.