Actually Red Mage is easier then Dancer but both jobs (Dancer and Red) are at the bottom of their categories.
Ease of play should not be a sole decision to have jobs do less damage. Generaly speaking most players do not want easier to play jobs, SE decided that on their own.
All 3 contribute in some way to parties (DNC, Bard and Mach) so no reason not to tune the dmg of DNC