It's a multiplayer game and people instinctively want to beat others in multiplayer games; they want an objective mean of being able to say to others "see? I beat you"; if that is missing they lose interest very easily.
Tanking and healing heavily lack that; DPSes can be considered bad if they only look at meters but they are the only ones who CAN use meters in some fights ("Patchwerk" ones); tanks almost never have that.