Technically the whole concept of 'tank' as we know it is really only a recent thing anyway, given the fact the earlier FF games didn't feature an enmity mechanic in gameplay (well, except for FFT that is, and that wasn't even a true enmity system but more just 'enemy goes for the weakest character in the party'), so a Dark Knight 'tanking' doesn't really count in say FFIV.

Really, DRK being a tank doesn't concern me as more I'm intrigued as to how the Job is going to work in regards to not having a base class - the only way I can think of how it could possibly function in this way is like ROG/NIN, it cannot be chosen as a starting 'Job' for new characters, but can only be unlocked after a certain point, and that upon unlocking you get the Job Crystal as normal, but equipping a great sword will automatically equip the DRK Soul Crystal as well and will not allow you to remove it, because there is no base class.

To be really honest, I'm surprised SE didn't take the easy way out and give it a base class called 'Soldier' in a thinly veiled attempt to rope in FFVII fans. But I digress...