Well i was sort of thought that too, but i would rather dps in cleric stance and switch back to heal tank back and dps again, than just stand there and turn myself fully into "HoT".
I was thinking to use it when im totally out of mana, but i really really try to avoid those situations, because then either way it's near wipe anyway and a bubble of HoT spell won't help.

I was using it lately in a1 when the boss jump up and everyone gathers as far as possible where boss will deal less dmg, i make a bubble so that... people get 1-2 ticks of heal before they run away. pretty useless but i save mana for 1 helios. I also liked that in a1 the boss jumps are pretty much every 80-90seconds, so my collective unconscious is always up for it. Thats honestly the only place where i found some little use of it.