From a DPS standpoint, 5% Job Haste isn't better than 10 Dual wield. That you have to spend TP to get it as compared to getting it for free makes it that much worse. Dual Wield II is only 15% Dual Wield, but that also means that you get more TP out of Store TP and Double Attack. I'd much rather have Dual Wield III, but at least there are small positives to go with the negatives. The larger reason I use /DNC is for the utility.
What I really like about /DNC is having access to Esuna (Healing Waltz), Stun(Violent Flourish), and Accuracy Bonus I. Box Step + Dia III is nice in plenty of situations and Reverse Flourish isn't bad either. You can also use an Excalibur and Haste Samba with no complications.
/NIN would immediately go back to the norm for me if /NIN had access to Innin. Almace would be used. Nukes would make mobs extra crispy. Life would be good. But, as with most things, SE balances as it sees fit.
