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There are no Tanks, healers, dps... Bull
Its true that there are no professions that can only heal, or only tank. There are however professions that can be tanks and healers.
Guardian is going to be one of the best healers in the game.
When you picking your abilities and skills, you pick the healing over dps/tank.
When you choosing armor/weapons, you simply pick items that have +healing on them.
Same goes for tanking.
So in a sense there are no true tanks/healers, however both are more than viable and will be required in some of the 5-man content.