You are conflating the group's overall goal (defeat opponents) with an individual role. A healer heals. It may often be efficient for the group for the healer to
also deal damage, but if the healer cannot fill the role of healing, then the group is doomed to failure. This is the same with tank. If the tank did no damage at all, that would still fill the tanking
role. It's a bit like VIT: if you don't have enough, you lose by default; if you do have enough, then it's you're free put efforts elsewhere in hopes of winning. The problem is that WAR is either incapable of filling the
role or requires far more resources to do so.