While a guardian's dps is not going to be that big in the overall picture of taking out a major raid encounter, it does play a role that is important.
Groups can be constructed to maximize MT agro and also to diminish the agro generated by others in the raid. The set that is all MT agro has to be at least as large as anyone else's agro in general. We have tools to mitigate those troubles like reinforcement, rescue, plant when someone over does their dps. The bigger you make the MT's set of agro, the bigger everyone else's can be. The amount of agro you can possibly generate with taunts is really a constant. You apply taunts when they refresh and the amount of agro is basically going to be the same over time. It is with dps that great improvements can be made in the amount of agro generated. This is why some guardians believe that the buckler line is so strong.