It's nice and all... It's just probably not going to happen... Bungie said before that they don't want designated class roles like that. You would probably see posts like "Healers only" when looking for raid groups, and wouldn't really be able to choose how you play to some extent. Plus reaction time would have to be so high in most situations for a healer to actually pull their own weight. Most wipes would be "your fault". I feel like your DPS matters a lot more in any given situation than healing the person you happen to be next to.