I don't think a melee healer would ever really work in a raid situation, at least not while positioning for mechanics is still a thing. Generally there are only 4 spots safe for melee, and 2 of those are taken by tanks. Most groups split 4 melee 4 ranged, 2 tanks, 2 melee dps, 2 ranged dps, and 2 healers. That would reduce group composition flexibility.