I feel like that biggest issue is that dealing with numerous healers requires an amount of coordination that is unlikely to be found in many random groups. There's not a whole lot of disadvantages for healers to overcome. I think instant cast heals should be removed - that way if a healer wishes to heal, they need to stand still and cast. It's already a major pain as certain classes to get up and close to a foe and remain in melee range, so allowing healers to easily mend themselves and run around pillars in circles is frustrating. A lot more so if there's multiple healers involved.