It Takes One To Teach One
AA, in "A vision for you" somewhere near the end teaches that "obviously you can't transmit something you haven't got" (italics mine). To me, that means that if someone is not an addict at all, the words he will try to inspire an addict with - be they Torah-truth, or not - will have no real effect. They may inspire, but somehow, they will not bring about any change.
AAs discovered that addicts - the least qualified of humanity - can help addicts.
To me, that is why rabbonnim generally help for guidance only - it is the addict that an addict needs to actually get better.
And that's why, to me, the words of Rav Dessler (somewhere in chelek alef of Michtav M'Eliyahu) ring so true here, where he writes that mussar will only have an true effect on the students if the teacher has actually been koneh the madreiga he is teaching about, himself. He writes that he teaches nothing that he has not actually done himself.