I believe that self-honesty is an immensely practical asset. You may be in situations where it is prudent to lie to others (such as where those others have you at the point of a gun), but being clear with yourself has enormous benefits.
To the extent that you are not crystal clear about WHAT YOU KNOW, and WHAT YOU DON'T KNOW, you're probably ineffective.
Often the brightest people are the clearest about what they don't know. It is a good thing to preface many statements with "I suspect that...". What do you really know? Do you really know that the earth is round with the SAME certitude that you know fire burns your skin. Do you really know that Astrology is baseless? Think about this!
Many educated people in the U.S. believe everything that they hear about science. They might as well have been born dogs, for all the use they make of their rational minds. Judge for yourself, and examine your relationship to what you hear.
Most people are really foggy about what it means to know something. They think they know a subject if they have memorized many facts about it. What does it mean to know a subject? What does it mean to know a fact? Think about it, this requires self-examination.