I think the point of "don't use cure 3 as tank heal" was just that, it costs more, and has less potency, than Cure 2. So using it "As tank heal" is good thing to avoid. Of course there are situations when it might be good idea to cast it at the tank, if you want to heal more persons around them. But too many newish healers think it's upgrade to Cure 2 abd start using it as supposed even bigger single heal, which it's not. That's what was advised against here.