If you look at it this way, Paladin is also a Tank class.

If we examine Final Fantasy IV, Cecil went from being a Dark Knight to a Paladin, and much of the time was spent being the party member that absorbed the damage from the enemy the best, thus making him the Tank of the party (Besides Kain, and Yang). This being the case, it makes sense that Dark Knight would be a Tank class.

I'm all for it.