The GLD class should exclusively be a tanking class so it gives new players a chance to get their feet wet with the early Guildleves and Dungeons, so they can start learning with easy content, which are there for practice. If a job like Dark Knight gets implemented, then GLD's should only have the option to go DPS if they wish ONCE they reach lv 30. If GLD's have a choice earlier than that, then we run the risk of a GLD player DPSing and deciding for whatever reason at lv 30 that they would like to go PLD and tank instead. What would happen as a result of this? Having lv 30 dungeons be a PLD's first tanking experience. Would you like to queue up for Brayflox and see someone say "never tanked before"?

While it is true that this can happen if a GLD hypothetically sticks to FATEs and decides to start tanking at 30, the odds of it happening now are much lower.