I know this question has been asked in the past and the devs refused to comment on the details, but a lot has changed over the eight years this game has been around and I think it's time to ask them again. I would like to ask the devs exactly what treasure hunter does.... not with a response that "it makes stuff drop more", but exactly what the different treasure hunter thresholds do.
A lot of the strife revolving around the Thief job is the ambiguity of the drop system and the meaningfulness of our "Treasure Hunter" ability. In fact the drop system itself is a mystery. I have heard people who believed that every item has a specific chance to drop and when a mob dies the game rolls a random number to determine if the item falls based on its drop percentage. They believe that each level of treasure hunter allows a "reroll" of the die, so if an item has a 10% chance of dropping then with Treasure Hunter 7 the game will random up to seven times until either the item is dropped (resolving in the item dropping) or all chances are used up (The item does not drop). A second belief is that each item has a set percentage to drop with no TH and each level of Treasure Hunter increases the actual percentage of the item dropping, but you still get only one roll of the random number generator when the mob dies (in other words if it had a 10% drop rate Treasure Hunter 7 may make it a 40% drop rate)
Players have tried to test treasure hunter for years. We used to think the job traits and equipment bonuses were separate in strength ( TH 1 and TH 2 job traits were commonly accepted to be vastly stronger than TH 3 (knife) or TH4 (Knife + AA), and so players used to say TH4 was actually TH2 + 2, or Treasure hunter 2 job trait plus a 2% bonus (hands and knife each equating to a 1% increase).
Now we are playing under a system where the mob's treasure hunter level is visible in the chat log and we still can't determine how useful higher levels of treasure hunter actually are. As far as the eye can tell there is very little difference in noticeable drop rate between a mob inflicted with Treasure Hunter 6, and a mob with a whopping Treasure Hunter 12. For all we know drop rates have a cap like everything else in this game.
S-E changed the rules of play when they made Treasure Hunter show up in the chat log, but it really hasn't helped the job's image because we have no way to measure what these numbers actually DO. I must reiterate that it is extremely difficult to tell the difference between a mob procced with Treasure Hunter 6 and a mob procced with treasure Hunter 11. It feels as if the entire worth of our job is based on the outcome of a single roll of the dice.
In summary S-E's vision for thief revolves highly around our coveted "Treasure Hunter" ability; so much so that in order to balance this all powerful ability they have nerfed much of our other potentials to preserve balance. If the devs at S-E feel this ability is so powerful and important I think it's fair to ask them for a more clear definition of what the ability specifically does.