The fact that they have learned to use so many weasel words (about, evidently, perhaps, etc) --and that they continue to remind us that they are imperfect and fallible-- undermines their credibility significantly. They do this because they want to have a ready answer anytime someone points out all the occasions when they got something wrong.
The problem is, if you are promoting yourself as the direct line to god and god's only channel for communicating his wishes to the world, you cannot get anything wrong. You might be imperfect and fallible men, but you claim to receive direct guidance from a perfect and infallible god. Is Jehovah giving you the wrong information time and time again? Or are you unable to properly receive his message and are misleading the flock with your mistakes?
If god is perfect and reliable and you claim that he speaks through you, you cannot get things wrong. So either god is constantly misleading the flock, or you are not god's mouthpiece. I don't really see an alternative.