I agree that the sacrificing HP for strength is not a good idea. Soul Eater killed many Dark Knights in XI, and healing intensive fights like Titan or Ifrit X would not be friendly to a class like that. Dark Knight should be build around absorbing stats like draining strength or defense.
Also, sure you can flip the argument, and it's quite valid about personal preference. But if you redefine it, one side is arguing for a completely original class + job to add more variety to the game while against the addition. There are already monsters wielding scythes (Demons), so like it or not, they're already in the game being used as weapons. Now it comes down to why shouldn't players have access to them and build an interesting class. The other important argument is it is a role playing game where people great their characters. I'm not saying great swords should not be implemented. I am always in favor of more weapons and jobs to open up the world for people to play how they want to play.
