Fact: A non-taru using a latent ring still has more/similar HP to a taru.* By this logic, all taru WHM must be worthless risks to the party.
You said it, not me. I cannot explain how happy I am as a mithra, non-taru white mage. I've survived dozens of attacks that have left me at less than 100 MP. It's basically an everyday occurrence.
The problem with all the "as long as you know you won't pull hate" arguments for using the ring is that you DON'T know when you'll be pulling hate. If the tank just drops dead, through your inaction or not, you may be unpleasantly surprised to find yourself in second place on the hate list simply by doing your job. And this is without considereing all the blindsided AoEs or getting a hate spike from healing that you could possibly predict and are a regular occurance for any serious WHM.
The inescapable auto-refresh also makes using the ring a burden. Doubly so for me, since I largely fulltime marduk hands with the regen effect. Having to play with macro swaps to get your HP latent to activate takes time and attention away from doing the parts of the job that often require lightning reflexes.
You can't even rely on stoneskin for added protection because of sublimation. That 200+ stoneskin you think you have is constantly being eaten away at. It might offer you next to no projection, it's really hard to say. The only way to be sure is to be constantly recasting it, and, not only does that again take time and effort, but it also totally erodes the MP savings the medicine ring gives you in the first place.
The only times this danger isn't true are on activities that are so basic that you don't need the ring. Or, said another way, the only time you can safely use the ring is when you don't need it.
I have a friend on game who, I am not ashamed to say, is a better white mage than I am. He's used this medicine ring as much as he could, and it's gotten HIM into trouble. If one of the best healers on the entire Sylph server that understand the job like only a totally obsessive can... if he occasionally gets killed trying to use the ring, than I make absolutely no recommendation to the average, run of the mill player to use it. You're putting your life, and the lives of your parties and alliances, in jeopardy all to see slightly higher epeen numbers.