In many situations, no matter how rational the person, imperfect information can lead to two choices appearing equivalent. How is a perfectly rational being supposed to act at that point? Making a decision based on some kind of tribal affiliation ("Well, these two things seem the same, but this one is how it's been done in the past") might actually be a useful fallback behaviour, as past successful experience might be an insight when all other things seem equal.
It's interesting to note that many of the behavioural patterns that are labelled as "mental illness" often combine "difficulty making decisions" and "lacking emotional response". People who are excessively rational often have trouble functioning in society.
Fundamentally, I believe that those deep seated irrational axioms are necessary for us to function. Without them, we'd freeze and crash at the next logical paradox we came across. Without a little bit of irrationality, how do you decide what to do next? What do you do when you get to the end of the decision tree?