Sure, Slayer could have done it alone, but why not add another spicy character like Graendal? I don't understand why everything has to have a point, and why it can't just be accepted.

For instance, Egwene could have been given the task of defeating Mesaana with the Wise Ones. Why include Nynaeve? Because it adds excitement. And I'd answer that for any number of scenes where characters seem to be redundant. I'm all for adding Forsaken into any scene if its going to create uncertainty and good reading.