Originally Posted by
Estellios
Once you get into some endgame situations you will see that Conjurer is far and away the only one suited for a main healing role for serious battles and is a necessity. In XP and small groups the damage isn't too much for people to recover through themselves (and it should be that way) but in Darkhold/Ifrit/Moogle the extra potency CNJ gets as well as Cura and Curaga make a gigantic difference.
CNJ is able to nuke because fulltime healing and nothing else is impractical and it would severely hinder the class in solo and small party situations where people need to cover multiple roles.
Betelguzah has been giving you valid reasons. Scaling spells removes clutter, makes it easier to balance level-capped content, makes it easier to balance multiclassing, streamlines battles, and is the best way to ensure that abilities stay useful throughout the entire game's life.