The point is to have different flavors. That flavor can be achieved through different mechanics and rotations. Not through something as fundamental for balance like overall mitigation or damage. The optimal goal for dps classes is also to achieve similar dps but through different flavors (which includes indirect dps by buffing other party members). One doesn't say what is the point of different dps classes because they all do the same dps.
Now if you need me to break down why having similar mitigation and dps is so fundamental for tank balance I could do so for you.