Thief Sub-Jobs

Experience Point Parties

The most common subs used in XP parties, and the reasons for using them.

Warrior -
Often the Support Job selected by those who have not yet unlocked Ninja, Warrior is one of the most common support jobs used by Thief. This support job offers a number of useful job abilities and traits.
Berserk is gained at 30, Double Attack at 50, Attack Bonus at 60, and finally Warcry at 70. Each of these increases the attack power of a Thief, especially on Weapon Skills.

Ninja -
The most common Support Job for Thief. While this combination offers nothing before level 20, little before level 24, it complements THF's role nicely from level 24 to 75.
At level 20 gain the ability to Dual Wield. A low levels, it allows a Thief to equip two different types of weapons (commonly Sword and Dagger) in order to utilize Sword's stronger WS Fast Blade while off handing dagger to compensate lower TP gain rate from lower innate Sword skill. (This is typically done until Dagger's Viper Bite WS becomes available. The offhand weapon can be used to increase desirable stats, such as dexterity, agility, or accuracy. Also, Dual Wield adds an extra hit on Weapon Skills, increasing the damage dealt on those.

When selecting weapons and gears, it may be worthwhile not to create a setup where integer number of attack swings add to 96-99%TP, forcing a THF make one extra swing in order to reach 100% TP. (The formula for calculating TP gain can be found on the Tactical Points page.)

With Haste gear and spells, 150-190 delay daggers, and Dual Wield, a Thief attacks very fast. However, even with the Subtle Blow trait granted by /NIN, the increased attacks will give monsters more TP per damage dealt compared to other jobs which use higher delay weapons due the TP gain for monster differ from TP gain for players.

Utsusemi: Ichi is gained at level 24 and makes a wonderful addition to a Thief's bag of tricks. Thieves are commonly used as the puller in parties, and the ability to negate three attacks makes this job much less stressful. Utsusemi: Ni becomes available at level 74, giving another three copy images with which to avoid attacks, at a faster cast speed. With A+ rated evasion, both the Utsusemi Ninjutsu, and status bolts, a Thief has amazing survivability, making this an ideal Support Job for soloing a variety of monsters as well.

Monk -
A Monk Support Job can be beneficial to a thief, especially before Viper Bite. When used with Sneak Attack, Combo can be devastating at lower levels, despite an E rating in Hand-to-Hand. This also offers the job abilities Boost at level 10 and Focus at 50, increasing a thief's attack and accuracy respectively. Dodge, gained at level 30, can also be used effectively with a thief's already impressive evasion to help with pulling.
Somewhat more situational and less versatile than either a Warrior and Ninja Support Jobs, this is never the less an effective combination in some levels/situations.

Ranger -
Whilst a thief's ranged attack skills are not very high ranked (C- in archery and C+ in marksmanship), they are none-the-less extremely useful. A ranger sub capitalizes on this fact, and can actually be even more damaging than a warrior sub in certain level ranges.
Until a thief gets Sneak Attack at level 15, they lack any way to do any serious damage to a mob. By subbing ranger, a thief can use ranged attacks to greatly increase their damage output in the low levels.
A ranger sub also complements a thief's common role as puller in a party. Wide Scan can be of use in certain circumstances, although it must be used correctly. Alertness can also make the difference between aggro and strolling right past a mob or potential link.
Access to ranged weapon skills is gained from a ranger sub, although in parties a thief should generally be using their dagger weapon skills. Sharpshot allows a thief to exploit the high TP gain per attack of ranged attacks, and the Accuracy Bonus traits gained at levels 20 and 60 also help increase a thief's TP gain nicely.
Another sub that works very well at low levels, and can be used very effictively even after that point.

[edit]
Specialized/Niche Situations

White Mage -
This is a good choice for getting around in dangerous areas. Being able to cast Sneak and Invisible on oneself instead of relying on medicines or Shinobi-Tabi helps to save money and inventory space. Also a good choice for when coffer hunting for Artifact Armor or for profit.

Black Mage -
Quite honestly everyone should consider leveling Black Mage to least Lv.17 for use as a support job for transportation purposes. Warp allows players to return to Home Point without having to waste Conquest Points or Gil on Instant Warp scroll or Warp Cudgel.

Samurai -
This is mainly for HNM, Gods and Dynamis. The ability to skillchain on demand with Meditate is the most important use of this as a support job.

Dark Knight -
This is mainly for HNM, Gods, and Dynamis. Stun is one reason for subbing DRK. Last Resort and Souleater, coupled with Sneak Attack, Trick Attack, and later Assassin's Charge, can lead to enormous damage hits and powerful Weapon Skills for piling enmity onto the tank.