Swiftcast is really good for Raise/Ressurection, that take very long to activate and sometimes you do need it quickly, but I don't think it's wise to spam healing spells because you pull hate to yourself and that would only get worse with Swiftcast having no GCD.

And maybe I didn't understand well, but isn't ATB much like the GCD? I mean, I've played many games with ATB and they were like a bar telling how much time your character have to wait to take next action.

And like someone else said, enemies also have GCD and casting time.