That's a legacy of the original version of FFXIV which ARR is a reboot of (version 1.0) - for that the Armoury System (the class and ability changing system in the game) was designed so that the player would have full freedom over what they did in the game - it was meant to be more laid back in that the original development team (not the current team that made ARR) envisioned that the player would start on a melee class like Gladiator, kill some enemies, then switch to a mage class like Conjurer to help out a friend in a fight, then during a break would switch to a harvesting class like Botanist and farm materials, then finally switch to a crafting class and make items to use or sell, all by simply equipping the relevant weapon or tool of that class (hence why each class only has one or two weapon or tool types associated with it).
The above was pretty much how SE described the Armoury System on the original FFXIV website (now long gone) and how they suggested players use it. Of course this has been changed considerably with ARR, but the basic idea is still there, in that once you unlock the Armoury System fully and join each class's guild, you're free to switch to any class you like, regardless of whether it's a Disciple of War (melee fighter), Disciple of Magic (mage), Disciple of the Land (harvester) or Disciple of the Hand (crafting).
