If you actually look at the various incarnations of Dark Knights over games other than XI, they've actually been a very tanky class:
The very first Dark Knight is Leon from FF2 who is a one of your most durable characters, with high hp and defense. The only "damage dealer" kind of thing he gets is access to Black Magic, which is no more anti-tank within the confines of the classless FF systems than PLD getting White Magic.
Cecil, in FF4, is *still* super tanky while a Dark Knight, what with high defense, hp, and heavy armor. The only "damage dealer" aspect he gets is the whole sacrifice-hp-to-increase-damage/deal-damage thing, which still doesn't preclude his tankiness given the fact that he is still your most durable character *and* it's another of the FF games where you don't have anything resembling a trinity construct so even the durable characters are supposed to be throwing out as much damage as possible.
In FFX-2, Dark Knight is, once again, a highly durable class: high hp and high defense. The same explanation of hp-sacrifice as applied to Cecil applies here.
In FFT (which the devs seem to be drawing a lot of their inspiration from), Dark Knight is *explicitly* a tank class: heavy armor, shields, knight swords, and an at-will hp regain ability that makes them virtually impossible to kill. Gafgarion is one of your best tanks outright.
The only incarnation of DRK that is an explicit damage dealer rather than following the model for tanks that the devs have been drawing from to turn into tanks for ARR is FFXI, which is nowhere *close* to "all games". At worst, DRK was a tank that used BLM instead of WHM. In general, it was a tank that had the option to sacrifice hp to become a damage dealer for a short period.
It's important to keep in mind that in every single Final Fantasy, with the exception of XI, *there was no trinity*. Everyone was expected to deal damage and take hits. If your only definition of "tank" is "has cover" then the only tank that would be around would be PLD, which is a *terrible* idea for an MMO since having only a single option for a role is boring as hell. When you're pulling classes from games that don't have a trinity and assigning them roles to fit *into* a trinity, you have to look at attributes other than "takes damage for other people" (especially given the fact that oftentimes the characters aren't actually meant to be balanced against one another); you look for high defense, high hp, and a lower than average damage dealing capacity compared to the explicit damage dealers (Cecil as DK compared to Edge or Yang; DK dressphere compared to Gunner or Thief; Dark Knight in FFT compared to monk, lancer, archer, or ninja).