That's how new tanks see it (minus the boss positioning because many tanks never learned that), because they don't use any of their mitigations or other abilities, but once they learn to use their full kit and protect the party, it is a very fulfilling role and you see how versatile abilities such as Blackest Night, Intervention, Nascent Flash or Reprisal are.
Let's consult SE for their definitions.- Tank: These classes and jobs utilize various defensive and enmity-gaining abilities to keep the enemy's attention, preventing other party members from taking damage.
- Healer: These classes and jobs utilize various curative abilities to restore the HP of party members and remove status ailments.
- DPS: These classes and jobs utilize various offensive abilities to dispatch enemies as quickly as possible.
So even by SE's definition, defensive abilities to prevent party members taking damage are indeed one of the responsibilities of a tank. It only mentions that healers should heal and use Esuna and that DPS should use offensive abilities to attack; it does not mention that healers or DPS should use defensive abilities at all.