Having to press 3 keys mid combat for a single desired effect, not to mention swapping focus from your main target at the time to target another just for this, is just not optimal. DnJ is an oGCD. With a Macro I can swap to who I want, when I want, even in the middle of a Gnashing Combo with minimal fear of clipping my GCD so I can maintain uptime and maintain damage on the enemy. I guarantee I can do faster with a macro than if I was doing it manually. It's why people use macros in the first place, across almost every job, with almost every player. It's faster and more accurate in specific situations.
Idk if you have looked at the leaderboards, or looked at the tournaments that have been happening, but there aren't many GNB's. Like, I am one of about two-ish on the Aether leaderboard. Like I said, GNB is the equivalent of DRG or SGE but for Tanks. No hard CC, but a lot of damage. That's the balance. You're "super powerful Aurora" comes at the cost of doing much less damage than either DPS or Tank stance, while having less sustain than Tank stance. Besides, you don't really swap to healer stance for Aurora, you swap to heal stance for the AoE heals on the oGCD that can be very good in very specific situations.
The healing boost is so minimal, I am curious what you have ever seen as "overpowered", because in over 1k games on GNB, I don't agree. The ONLY instances where the healer stance is good to have in in heavy melee comps, usually on Lava Map, or in Overtime. Situations where you can hit the most people with the AoE healing that is the main reason why you would go into healer stance in the first place. And even then, your damage and personal sustain are gutted, hence why it is balanced.
Not to mention GNB is far from being the best tank at the moment. PLD and WAR are by far more consistent in what they do. Their utility and CC carry harder in more situations which is why they are the better pick most of the time. IF they made GNB able to choose stances as they please, while keeping what they have now, at most they might actually be on the same level as PLD and WAR are right now. Your damage wouldn't change, other than the availability to swap to DPS stance more frequently, the heal stance would still be very situational and a 10k heal doesn't do much against a 16k Flare, 20k Drill, 22k Wyrmwind Thrust. At most, it would just make GNB more CONSISTENT, which is it's main issue at the moment.