Next expansion, please make PLD optimal MT and WAR optimal OT

[TouchandFeel]

I have two issues with what you have presented.

First is the main argument presented that jobs should be optimized to fit into sub-roles, Main Tank and Off Tank. All that does is pigeonhole players into playing specific jobs depending on whether they prefer to OT or MT, ultimately limiting player choice and with only 3 tank classes to currently choose from we need all the choice and variety we can get with what little there is.
All tank jobs should be able to shine as MT or OT, relying on different game-play styles to shape how they achieve that.

Second, making DRK the "inbetween" tank just results in it being sub-optimal in both roles which would be terrible for the job. WAR would be a better OT and PLD would be a better MT. Sure, DRK would be a better MT than a WAR and a better OT than a PLD, but that really wouldn't matter when the optimal thing to do would be to bring a PLD as MT and a WAR as OT, resulting in DRK just being left out since it would not be optimal for anything.

Besides as others have pointed out, it's best to not have a dedicated MT and OT and to switch off to best utilize the cooldowns and abilities of both the tanks.