Then pick them under scenarios.

SMN can help you on runs you know you have to deal with a great deal of enemies or just want to play ranged DPS instead of melee.
WAR is a tank, so of course you just have to keep it for the moments you wanna/need to tank.
I can see the problem with DRG - NIN tho, since they are both melees... but alternating can give quite the fresh feel.