The question on Tera is how do you keep the action, you against the boss feeling without making classes useless or without making full on Trinity 0 action combat. We notice in your response you made little to no sense. If your tank holds agro and is meant to get hit since they can not dodge like the faster lighter armor people.
If you use the tank healer dps method and the tank holds agro 100%, dps can do damage how does the game(buy tera gold) feel action and fast paced when the mages have to really do any dodgeing. The healer spams heals and only runs on occasional boss moves. If you go party skill and you ditch the tanks trinity system agro, your party must be good at dodgeing attacks and fighting a boss like you have to in action games then whats the need of the tank, for that matter the healer if its only there to fix your mess up dodges.
If 1 dps has agro and is dodgeing and not getting hit the other dps could be doing full damage. How is that not faster than a low damage tank holding agro getting healed by a low damage healer. If the tank and healer are not low damage then their survivability can not be to much higher than the other classes.