My hope is you won't have to be a TANK role to actually tank. Perhaps jobs with heavy armor can tank, although not as effectively as jobs designed to TANK. Same with healers. Otherwise there is bound to be a shortage since there is only 1 dedicated healer at the moment and the armoury system forces traditional white magic (Cure, Protect, Regen, etc) into WHM-specific thereby not allowing other roles to use it. Guess that all could change with rebalancing.