The answer is simple: it's efficient. There are several reasons.
- It reduces the risk of overheal and i can make use of alle my skills (instead of spamming cure II for example).
- From a whitmages point of view i have benediction and tetra. The more health a tank has, when using benediction, the more healing capability of this skill is wasted. It's similar for tetra, using this while the is over 50% might waste capability.
- Healing a tank below 10% boosts the limit gauge.
- Getting into cleric stance may be a risk, since it has a five second recast, in which i can not deactivate it. If the tanks health drops too fast during this time, for whatever reason, i'm caught in cleric stance and can't heal properly. For that reason, i wish to reduce stance dancing. This means, the tank has to drop lower in hp, before i start healing.
- When you know your healer class and the instance well enough, the point where you have to start healing gets lower.

Tanks shouldn't take this behaviour as a healers attempt to offend them. It's more like testing our own limits, so we get better and play more efficiently, which might be helpful in harder content.