Joshua, bias is by definition faulty reasoning, hence if it is systemic it is a form of irrationality. As Redlawsk put it “these affective biases may easily lead to lower quality decision making.” Thus motivated reasoning is presented as a systemic form of irrationality. I disagree.
If you don’t like the term irrationality then let me just say that I do not think it is a form of bias, nor that it leads to lower quality decision making, in the normal course of events.
Of course extreme cases are a form of insanity. But so is extreme cleanliness or any extreme version of normal behavior.