I decided to take on the role of astrologian for a while and I have this to say: there are as many bad dark knights as there are any other bad tanks.
I've had great dark knights that mitigate damage right (actually use their buffs and know WHEN to use them), as well as the opposite. But that is also true for the paladin. All in all, might be there were more 'bad' dark knights at the start because it was a new job, but right now it's at the same point as any other tank I think. Fix the horrible bugs that are plaguing some of the skills and it will be golden - no need to further adjust I don't think.