Pure support or CC classes would never work in ffxiv due to the game design. They would have to up min party size to 5 to even attempt to do it. And yes Support would be a required class to have if they did it correctly.
Dnc in FFXIV is about as good as we are going to get a dps/support hybrid more or less.
As to "why?" it is a major archetype in mmos. You have basically 5 archetypes: Damage Dealer, Healer, Tank, Crowd Control, Buff (Support - depending on the game sometimes lumped in with CC). It wasn't until the WoW style mmo came around that the last 2 archetypes started going away or being combined with others.
Alot of people enjoy that kind of class. The ability to make the party stronger and more efficient was both enjoyable and important back in the day. Example Brd in FFXI increased everyones dmg output significantly, Reduced the tanks dmg taken significantly, worked as a battery keeping the mages mp up so there was no downtime, CCed packs incoming mobs so quicker kills and larger pulls were doable. Thier dmg output was well non-existant though. Rdm in FFXI was a support/CC hybrid class as well. Subpar healing, really subpar dmg. But best CC in the game with the ability to support heal, Haste to increase dps dmg, and refresh to keep everyone's mp topped off.
Back in the day in FFXI the setup for a standard party was Tank or War/Nin, Healer/Rdm, Support, Dps or support, Dps, Dps. In raids it was (Party 1) Tank, Tank, Healer, Rdm, Support, Support (Party 2 and 3) Dps, dps, dps, dps, Rdm/support, Support. No possible way to make it work in FFXIV's 4 man parties. In raids or 8man parties it very well could work... but would not work with the leveling aspect of the game. Example, Can you imagine soloing on brd if all they had was a dagger and auto attack + say a 100dmg a tick song debuff for dmg output.
Honestly it is a problem with the whole genre, with everything being homogenized. Support and Crowd Control classes were a good aspect of the game back in the day.