Healers play a vital roll in not only the health and performance of the Tank, but the entire group. With a constant stream of healing spells and (usually) large stores of mana to draw upon, the Healer can often turn an impossible situation into a large chunk of experience. Also, Healers serve as a mid-run speller. When Sanctuary falls, the Healer is generally the first to renew it. When a Hitter needs a quick Frenzy during battle, they call on the Healer.
While all Clerics are Healers, not all Healers are Clerics. One musn't overlook other classes skilled in Read Magic and Arcane Knowledge, as they can also provide health for the group in the form of Brandishes. Just remember, a Wizard that's Brandishing healies is generally less effective than a true Cleric, so try to cut them a little slack.