I don't think the problem is in the game system. The reason people always end up with these same stupid nonsense builds is because they are effective given the games stupid combat AI.
Imagine this party is facing an opponent controlled by human, like me, I would just walk past the tank and go for the mages and rogues (who have low defence because they dumped their con and res). I wouldn't care wether the disengagement attacks hit me because the tanks' attacks are so weak they are pose no threat at all.
what the devs can do here is to improve the AI, so that the enemies reacts in a more realistic way. This will encourage the players to create more balanced, sensible and varied character builds to deal with the improved AI.