Healing isn't really needed. The game is designed to have the fireteam work together to take out enemies before you get killed. Rather than your buddy healing you after you get hit, better that he takes out the enemy about to shoot at you and you never take damage at all. Also revives are super easy and there are guns and perks that can heal you. We do just fine without a 'healer class' as it is and having one could potentially unbalance the game. Imagine fighting in the crucible against a team of enemies that all heal each other non stop. While it's a nice thought, healers would just cause more problems than they might solve.