Except that's not how it works. The moment one class becomes superior to others for specific content, people will stack that class and give everyone else the middle finger. FFXI saw this. WoW saw this. I'm pretty sure Lineage II saw this, too.
Variety is already pseudo-encouraged in group comps because of the small penalty to building the LB gauge when having multiples of the same class, not to mention keeping the loot pools from boss mobs in mind. At the same time, having more than one of every job is not detrimental to the run, so we're actually in a very good place design-wise. We can have a "one of each" comp and a "some repeats but roles are covered" comp and still be able to clear content. Why you would destroy that so that a few can feel unique and special is beyond me.
I love you for this.
PS: Mechanics are the best place for variety between jobs and classes. The fact that MNK has greased lightning while DRG has relatively unforgiving positional requirements does make those two play and feel differently despite being competitive against each other AND fulfilling the role of melee DPS. PLD and WAR actually are very different because of the approach to tanking each take. BLM and SMN also are distinct from one another without making groups take one over the other, and that is a good thing in the long run.