If it were only the tank personal damage suffers from the group performance, it isn't too bad; the problem is that the group could easily suffer from the tank being bad and refuse to switch between stances "because of damage stance deals bigger numbers".
Tanks' damage stances being removed is similar to how they attempted to fix Cleric Stance on healers and gave up, if you want tank mechanics, please consider asking for smaller target circles, more freedom for tanks to position bosses and leave enmity alone.