Dark knight being a tank isn't really that big of a change. Even when designed for damage output they were still tanky with high VIT and heavy armor. When not utilizing job skills to optimize damage DRK in FFXI was actually only second to PLD in regards to base defense due to armor available and VIT. Ninja in FFXI was actually designed initially as a damage dealing job. Players chose to use it as a tank after discovering how useful utsusemi was. Hence why NIN doesn't have that spell now most likely.