I think every job is been excluded at some point, aside from DRK and NIN from what I can recall.

At one point:

BLM got excluded from Titan EX because of heavy movement.
SMN excluded because BLM just plain beat it out in damage.
All of the above plus Monk and DRG got excluded from Garuda EX because stacking bards was safer, even if it was slower. (before LB gauge filled slower with stacked jobs)
WAR was excluded because PLD was better at taking damage early on.
PLD excluded because it's abilities aren't suited towards certain content like A4S
WHM excluded because SCH shields are amazing and fairy buddy helps a lot.
SCH excluded because stacking HoTs on WHM was great plus the burst heals
BRD was excluded from groups for being the lowest DPS despite it's utility and Foe
MCH excluded for low DPS
AST excluded for low healing output and buffs from SCH being better and more consistent.


Balance changes, usually for the better. Sometimes it makes things overpowered, other times underpowered.