Since you could only DA+Abyssal twice, it would have been, on average, a combined 120 potency cure from a single target at the expense of all your MP (0-240). 200 is probably good enough to heal decently and it's costing no MP.

DRK self-heals haven't necessarily been a problem despite not being as powerful and freecast as other tanks.

TBN has a 15 second cooldown. Being a shield that absorbs damage makes it part of self-sustain like an SCH shield's impact on survival. You have to use it right or you lose the MP needed for Shadow, putting your ability to keep Darkside up at risk.

Magic mitigation is fine because most raid casts are magic damage. It's helpful but not meant to be required anyway if healers have it under control. It'll be balanced because the other raid tank should not be a DRK and all other tanks can mitigate physical. Opposite is true for gunbreaker.

DRK has all the cooldowns it requires as a tank: Rampart, Shadow Wall and TBN (so Dark Mind is an optional/for fun buff). Can use it if you know casters or magical enemies are attacking you with things like aero.

If they copied the other tanks they'd get accused of homogenizing them. Oh wait, they are getting accused of that too. They can't win, can they?