-One of the main selling points of the game is the hability to play all classes in one character.
-One of the classes with has to choose between two completely different roles to play effectively in endgame scenarios.

Classes are an old concept that just doesnt work, remember that jobs were released so that the game could be changed into something resembling a FF title without completely losing all the progres we had made so far in 1.0.

The ones who suffered this the most were Conjurers who suddenly became healers and thaumaturges who could no longer heal, yet SE decided that the class concept should be kept for lore and adaptavily (Imagine having to rework the entire city, questline and NPCs by elimitating the classes in 1.0) so they kept them and since progress was saved to ARR they still exist now,

The prupose of "Using class for solo content" just doesnt work for one reason, there is no such content in ARR,