The numbers for Shinryu are probably most telling; DRK has an 'advantage' in clear statistics for Omega since people used it for prog and many groups just stick with their prog strategy/compositions.

Right now PLD and WAR are the #1 and #2 most popular classes for Shinryu clears, with DRK sitting at about half the clears of either, which is a pretty piss-poor showing considering that Shinryu is actually a really "good" DRK fight with plenty of opportunity to use Dark Mind.