Facts:
- Monsters have set amounts of HP
- Events have time limits
- Damage done / time = DPS
- Monster HP / time remaining = required DPS
- If DPS < required DPS then you lose
- The higher your DPS, the better your chances of winning
It's pretty simple really. Unless a job increases the groups DPS through other means (enhancement spells, curing, stunning, etc.) more than just bringing the job that directly does the most DPS, then bringing the indirect DPS job only hurts your group.
People bring those jobs when they are testing things and then once they know the ropes, they maximize efficiency.