Depends on whats being done. I remember quite clearly in velk's lab keeping my group healed with only the ability/time to heal with the velious HoT spell, clickey healing legs, and a nerfed donnals breastplate.

A nerfed Donnal's breastplate with a 30 second cast time and a 7 minute cool down timer is surely not something I am going to DEPEND on as a Cleric. But every bit helps. The only real time I remember using it a lot was in raids after a rez, or last chance heal on MT in Raid when oom on Chain Healing. 400 mana for Complete Heal is not a ton with some good Velious mana gear on. But you bring up some good points. :D