Actually, Qunari Rogues aren't really much better than other races. Dual Dagger Rogues seem amazing, until you realize how their damage is calculated. See, all Rogue abilities use only the Main-hand weapon to calculate damage. The only 2 exceptions to this are Flank Attack (uses only the Off-hand weapon to calculate damage) and Twin Fangs (two hits, one using only the Main-hand weapon, the other only using the Off-hand weapon). This means that the Dual Dagger Rogue only gets the Vitaar damage bonus once, not twice. And before you point out Twin Fangs, two Vitaar plus two Weapon equals Vitaar plus weapon when calculating percentage, so you still only really get the damage bonus once.
The only way the Vitaar damage bonus becomes amazing is if you're only auto-attacking. Basic attacks alternate between Main-hand and Off-hand when calculating damage, so while each strike only gets the damage once, the speed of the attacks makes the damage add up very quickly. Problem is, if you're only auto-attacking as a Rogue, you're not doing it right. Especially if you're an Assassin; Hidden Blades and Mark of Death can put out so much more damage than auto-attacking ever will.
Of course the % boost is the same, no matter what ability is being used, but if anything it would be more noticeable on abilities rather than on autoattacks.
Hidden Blades + Overkill does 1800% damage total, and an assassin really should be critting every HB hit, so that's multiplied out by about 250-290% (crits are what people are talking about when they're saying it "applies twice"... again, this doesn't change the % damage boost, but it does double or triple the flat damage boost), as well as any dex / willpower / attack % bonus the character has.
So that 19 damage turns into 1000-2000 extra damage on Hidden Blades. Not a huge amount, but nothing to sneeze at. Enemies have cooldowns too, so a 12% damage increase can make a fight 15-20% faster - less chances for the boss to tack on guard or summon adds.