Here's the thing. When they are building a class or Job they balance it against the existing classes and Jobs then test their build as they think a player would. Given the developer's country of origin their view of how a player will behave is biased towards a certain style of play. But as non-developer players are won't to do, they are going to try and find every way a class or Job can be played. This is how XI's ninja turned into a tank and red mage/blue mage/dancer became the go-to Jobs for soloing. Unlike XI, though, the developers are keeping a close eye on the game to see how players shake out the classes and Jobs, which even after all this time still surprises them. Like healers that DD, who could have anticipated that? Clearly players found a way to squeeze even more damage out of Dark Knight in an unintended manner, and Square decided to fix it instead of letting it get out of hand.