While that could come in handy, PLD and WAR would still be able to it better, for free, and cover both types of damage. Doing fights at lower level would also make dark even weaker than it is already, since you don't have TBN there. In terms of party defense, I would rather have them give DRK a debuff based party mitigation skill that fits in with the retaliation part of the job (ahem previous Reprisal).