One reason is stats. When you pop on your cleric stance, if you had reasonable healer stats, you now have reasonable caster stats. Also, healer role naturally has downtime, nothing to heal -> free time. So healer can damage mana/time efficiently with proper stats.
Damage trying to heal however. First of your healer stats are scrub so the heals you can toss out are pitiable comparing to the time and mana they take to cast. Also damage role doesn't have downtime. If there are mobs to kill, kill the mobs.
If your tank keeps dying because lack of heals from your healer (or healer dying), -then- you might consider adding into the heals (or raising the healer. SMN are good for that) but if the healer is up and fine you just hamper everyone by not making sure the stuff dies quickly.