What could DRK have learned from FFXI?

[Reynhart]

Darkness Cecils one skill that they removed for the American release was so weak it wasn't even worth using.

Wow, that's heavy misinformation there. Darkness was a very useful skill...for AoE purpose. And the reason why it was removed was not because it was weak, but because they thought western people wouldn't be able to grasp the strategic use of character skills. Love the irony.

And outside 2 to 3 total fights in the ds remake you used it only once per those fights thats it.

You mean in the game where it's a toggle ? Sure you use it once per fight.

And for the record, in FF III, it's not a Dark Knight as in 暗黒騎士 but a Mystic Knight まけんし. It was only translated as Dark Knight in the DS remake...where it gained Souleater.

But boy darknight had "always used hp to deal damage" no it hasn't lol.

Yes, it has. XIV is the only exception in the main series. And how convenient to push aside every other Dark Knight incarnation in FF spinoff and affiliated games, where they do sacrifice HP everytime it's a job you can use.

Dark Knight attacks are fuelled by MP as a representation of their life-force instead of HP.

Which is a very bland representation because all casters are already "fuelled" by MP. At best, give DRK another ressource where using it all would be bad, not related to your HP...which, strangely enough, was close to how Darkside worked in HW.