Playing the game's story and general content unlockables (dungeons, new areas, features such a grand company or being able to ride a chocobo) can only be done as a disciple of war/magic - a battle class. Disciples of the land (gatherers) and disciples of the hand (crafters) cannot fight properly, and are not even allowed to enter dungeons and trials at all. While you could technically level all the way to 50 without touching anything else as a gatherer or crafter, you leave out the majority of the game's content off, still. Plus, as mentioned before, all of Heavensward's areas are locked behind story content, and you would be unable to gather there (level 51-60 gathering nodes).