That might be true for casual content (for queue time convenience), but for high end content the roles are indeed split up to have 2 tanks/2 healers/1 ranged/1 caster. And that's not a community thing, there are fights that require two ranged DPS, and the ranged and caster jobs are intentionally designed to not synergize with each other very well.
With that in mind, it makes sense why the Devs would want to add to the role type that is lacking.