From a article at NewScientist.com:
The research, by Don Moore of Carnegie Mellon University in Pittsburgh, Pennsylvania, shows that we prefer advice from a confident source, even to the point that we are willing to forgive a poor track record. Moore argues that in competitive situations, this can drive those offering advice to increasingly exaggerate how sure they are.
I've found this to be completely true in my experience doing sales as well as when giving advice to people on tech related stuff. People who I know, or at least think should trust me more will go with someone else who appears more confident. I routinely try and give both sides of an issue when someone is asking my opinion - but that, in effect, gives the impression that I'm not really sure what I'm talking about.
Obviously the same is true of television commercials and advertising. You wouldn't trust someone trying to sell you toothpaste if they said that 4 out of 5 dentists were pretty sure it worked well.
It's frustrating at times, but I'm not willing to become Mr. Douchey Loudmouth to win people's trust. Their loss, not mine. :)