As a PLD, stunning, stance changing or healing means interrupting our combo and losing one of our debuff. Doing our most damaging combo means not gaining our max enmity.
As a DRK, each time you spend Dark Arts on one skill, it means you're not spending it on another.

As a healer, removing a status means delaying a Cure by one GCD, the same if you decide to do some damage.

WAR is not that complex to play.