Being defensive is very crippling. It's hard to concentrate, you may feel angry, inpatient or suddenly tired and mind wanders away. Reading speed and comprehension slows down.
On the other hand, being aware and honest of your emotions and how they affect your thinking is important critical thinking skill. When you can realize your defensive attitude you can usually overcome it with effort.
If I remember correctly, high IQ people have less cognitive biases that affect formal reasoning, but high IQ don't reduce biases that distort the ability to question one’s own judgment. Ability to reflect own decisions is correlated with dispositions like "active open-mindedness."