There will always, always be at least 1 viable class from every archetype, because having 1 of each archetype increases the group's DPS by 1% just for having it around. Unless an archetype is so bad that it literally drags down the entire party's group more than it merely existing raises the party's DPS collectively, then you will see every group running it. And a class would have to be pretty bad considering 1% represents something like 750 DPS by itself collectively to the group.

And as far as I can tell, Dancer, arguably the worst class in the game, isn't bad enough to justify ignoring them in favor of a slightly more optimal comp because of that 1% DPS increase it brings just by existing. The same increase BLM brings, SAM brings, GNB brings, and WHM, just for existing.

They changed the group buff to encourage diversity, and while classes certainly need help, that 1% collectively is still huge.