Actually, consider this: Death Knights don't have the same level of Defensive talents as Warriors and Paladins, or even Druids, and also unlike Warriors and Paladins, they lose out on the major stats buff of having a one-hander and shield, which can be made up for by having two one-handers, but two one-handers means double the chance to get parried, and therefore take more damage. And yes, Expertise, but as a tank you can't give Expertise priority over Defense, cause then you just fail and die.
It is, however, now a mandatory rune for tanking, eliminating the ability to really choose between your runes. And then, Death Knights should never be using weapon enchants anyways (you know, enchants other than their runes).
^_^