The same can be said for AST, SCH, and WHM. Astrologian is being left out now for not bringing robust enough heals to the party.

Moreover, the way WAR is set up it provides the Slashing Debuff giving some benefit to both tanks whereas the PLD and DRK can't really complement each other. This makes WAR mandatory in raids, almost and the other tanks can be left out, in most cases the PLD.

Three is a crowd and I believe this is a bad design.