There was really nothing ever stopping WHM in XI from being a DD except that it meant replacing healing and MP gear with melee, which parties would simply /rage about if you even dared. That, and the fact that MP took so damn long to recover...
In fact Hexa Strike was one of the stronger weapon skills (even compared to real melees) in the game, at least at the 75 cap.
In other words, jobs are what the playerbase makes of them.