Tanks perform best when full healed, or at least when the healer maintains cheap buffs and HoT on them.
Watching the fight and waiting the tank lost 1/4 of his health before pressing a key doesn't look like an optimal use of the healer time. When they meet a slow healer, the tanks will trigger their cooldown early despite they're much more expensive than the regular heals (some cooldowns can be used once per pull or once every 2 pulls for instance)
Playing a healer, I don't see how I could run out of energy during pack fights by just healing the tank and the DPS. Healers might be short of energy when they don't use their regenerations powers efficiently, spend efforts to heal themselves from non handled attackers or when they DPS.
Depends really on tank. Shadows/sins, due to the problems switch spikiness are better kept filled up. My jugg has enough defensive cooldowns to survive till healer gets their heals of CD
also really depends on the fight. Sometimes trash pack (5 dogs in mandolin raiders when not CCed can hit stronger than boss
And a question about armor debuffs. Do they stack? If jugg, merc and sniper put their debuffs on the target, will the armor go down by total three debuffs or is the strongest rebuff counted only?