I agree with that to an extent, but for example think about it like this. Say you are presenting knowledge to another person, if you do not portray that knowledge confidently then most likely those who listen will get much less than if you presented it with confidence. I mean look at Obama for instance, people worship the guy even though a lot of what he says is absolute BS. However he has mastered speaking with conviction and authority therefore what he says holds more clout. Even intelligent people fall for that stuff, if you come off as passive it often does not matter what you are saying because people are already passing judgement on your demeanor.