Crit damage: WAR also has 8% from trait so it's actually net +5% for them. If you're including Kaftan, most Dancers shouldn't wear it in anything but capped-haste situations due to our AF3+2 body.
Dual wield: Dancer only has 5% more Dual Wield than Thief from traits, but our AF3+2 body is 10%. Our neck is matched by your boomerang, and we both get Suppa. The gear inequality is much larger than the trait inequality in this case, especially because we generally sub NIN anyway.
Saber Dance: Saber Dance (20% DA with 5/5 merits) is basically matched by Thief's 10% Triple Attack (5% base + 5/5 merits). Saber Dance is actually a little worse because it replaces job trait Double Attack from /WAR, which makes our native Dual Wield a little less important.
Haste Samba, a ton of STP, and Waltzes vs. SA, TA, TH, and marginally higher dagger and evasion skill <-- situational, but Haste samba and STP probably wins from a damage perspective.
People always say that SE neglects Thief. They want Thief to be for TH only. etc, etc. I absolutely think the opposite. SE initially based their entire battle system around Thief's Trick Attack. They created a handful of jobs capable of tanking and several jobs capable of briefly taking hate (through SA or TA) and surviving an attack round or two. Third Eye (and the stupid far-eastern defensive gear), Utsusemi, Super Jump, Counter/Counterstance. All not meant for permanent tanking in a Refreshless release-world with a paucity of Haste gear. The idea is that Thief briefly shifts hate around in the party to facilitate damage spikes. I'm sure many of us can remember a time when the game actually worked like this, and a THF with SATA Viper Bite was king (mostly because it doubled Attack, though we didn't realize it at the time).
Obvious problem is that the game hasn't worked this way in half a decade. I've entered and graduated college since the game worked this way. SE still hasn't come around, and gives Thief crappy unstacked WSs like Rudra's Storm.
Overall I would say that, while Ryan and Krab here (lol) would like Thief to balanced in terms of the DD hierarchy, SE is trying to balance the jobs in terms of desirability. Even though Dancer is a much better DD than Thief, they're less desired at the moment because they don't have TH. Treasure Hunter is a hugely desired job trait, and because Thief has it they don't need the DD advantages the other jobs need to see use.