Shaman

VahShir Shaman with Twice Blessed Staff from Temple of Veeshan

Shaman is used primarily for their utility benefits for a group:

• Epic click: Ancestral Honor (very high HP group buff)
• Kraken spells (adds damage proc to the group)
• Ancestral Grudge (increases attack and chance to double attack)
  
  

Even if you don't play a Shaman in your main group (it is likely wise not to :)), it may still be a good idea to level a shaman with your main group if for nothing more than the group HP buff they have.