We calculate how to get the most healing from your available mana pool. Increasing the mana pool is often a better use of stats than +healing to achieve this goal. In general, +healing only becomes better once you are in a situation where you are not running out of mana. That situation only happens on a short fight - you can always use up your mana to do more healing in TBC.
If you want to see +healing recommended over more mana, you would need to reduce the fight length or change the spell breakdown to use more mana efficient spells.