I see absolutely no point in have Oblation in it's current form. If other tanks can do just fine with role action Rampart and their job's 120s cooldown with lvl82 addition that bring mitigation and heal to deal with tank buster, then why does the DRK need that 10% mitigation without extra benefit while TBN can just do the job if one can well manage mp usage for it? Current state of the DRK has nothing special besides having a mp required barrier that hurts the mp spender in dps rotation.
If Oblation can allow much longer mitigation uptime by increasing either its duration, decreasing it's cd time, add something else to it, or becomes a better stronger TBN; then we can maybe call DRK's specialty as a tank that takes less damage than the other 3 tanks for a longer time.
PLD has physical/magic rotation and Clemency , GNB is known as dps tank, WAR is best at survival, and what does DRK have? You might say DRK is usable in 1v1 boss encounters, but that doesn't mean it's okay to be fed with leftovers while others received full set meal.