First of all I agree with you on raiddesign, but in this case it wouldn't matter too much.

Thing is, with 3 tankclasses, that the devs want to have one being more dpsy but squishier, one being the middle ground, and one being more tanky but having lower damage, they can't realisticly balance encounters in such a way that the higher dps wouldn't make the first class the prefered. Even if they increased the need for more "tankiness" they still have to tune the encounter so that the squishier tank can manage it, because if they make it so that this class would have problems handling it, it would just turn the balance issue upside down. They can never realisticly have higher survavibility matter, without outright making the other classes unable to complete the encounter.

Also, making the "tankier" class having such a huge survivability that you could outright have a healer (or even replace a healer) dps fulltime to make up for the tanks lower damage, would make the class unbalanced and suddenly the prefered choice instead. Balancing a class around having higher "eHP" than the other classes, is realisticly impossible, since it'd be next to impossible for the devs to manage to reach and maintain that kind of balance.

They'd be better off just rebalancing all the tanks in one sweep, trying to make sure that they have at least near the same damage output and survivability. Uniqness can be achieved through different playstyles, looks and feels of the classes.