While you can say that All DD jobs are within 5-10% of another you are not taking into consideration the special abilities and tactics that can be employed on a per job basis. Every job can excel over another job it just depends on the particular task at hand.

Name me something that a WHM can do that a SCH cant that isnt cure V/VI or Repose.

Giving other jobs stuff that makes WHM become less important than its primary function (healing) is going to force the dev team to add harder monsters and make WHM get new spells/abilities - in response to which you will complain about not being able to keep up with cures once again.

In hard battles you want a whm there. If you dont have a whm then either use multiple healers or find someone to fill that 1 spot.