At this point in WoW's evolution, there's really no purpose for "pure" classes anymore. Warlocks, Mages, Hunters, and Rogues are seriously harmed by not having viable healing or tanking specs to fall back on. The 7 hybrid classes have better access to dungeons, raids, and are more flexible in terms of overall spec design. An Enhancement Shaman for example is pretty different than an Elemental Shaman in terms of playstyle. Meanwhile a Mage, though having a wealth of spells and abilities, plays pretty similarly from spec to spec, especially with the Mage bombs being such a significant DPS contribution to every Mage spec in MoP. Warlocks also had this issue, but thankfully they were revamped in MoP (ironically by giving the class a psuedo-tanking spec).
I just don't see the point of having pure DPS specs anymore. Blizzard should seriously consider adding healing or tanking specs to the four pure DPS classes. If that requires a fourth spec to make it happen, then so be it. As it stands, pure classes are at a severe disadvantage to the hybrid classes.