Thief gets: Triple Attack (10%), Dual Wield I, Sneak Attack, Trick Attack, 13 Dagger Skill, Feint, Conspirator
Dancer gets: Saber Dance, Dual Wield IV, Haste Samba
Generally in this matchup I would say that Thief comes out ahead, especially if the Dancer is attending other duties besides offense.
However this match up is closer than it should be. Dancer is not supposed to be a solely offensive job, and given that I think it's damage output shouldn't be nearly as close to Thief's as it is. When Dancer was first introduced it was pretty well balanced. However it was given numerous damage related buffs and what has made so close to Thief is their innate level of DW they were given. Dancer having native DW IV gives them the ability to use /WAR and not give up any delay reduction. Thief having only DWI restricts them to staying /NIN.
I think Thief should get DW IV, or the two jobs should both get DW III. There should be a decent offensive difference between the two jobs and the best way to accomplish this would be to equalize Dual Wield and keep Thief's other offensive abilities intact to provide that edge.
Alternatives to adjusting Dual Wield include adjusting Sneak Attack and Trick Attack timers to be significantly lower (30~ seconds vs. 60). At the haste levels the game is played at now their timers are far too high. Right now the skill difference is insignificant in the majority of situations. Even against more evasive mobs I think we need to be at natural A+ with Dancer staying at natural B+ before skill really becomes a deciding factor.

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