People need to stop mixing "knowledge" and "intelligence". It's true that a magician can, will and have read and learn more books than the average person, and a barbarian might not know all the rule to live with more civilised person, but it's only knowledge.
Intelligence is knowing that even with all your knowledge, you might not be the more suited person for the task at hand. For example, dealing with a tribal spirit without getting the party slaughtered. Here, the barbarian who know this spirit have more chance to success than the magician who only saw it in books.