How well do you receive truth or words of wisdom when it comes from an unlikely source? If the village idiot suddenly professed to have secret knowldge of the universe, and s/he actually does, do you shrug them off because it's some nutcase? If a person of more status and credentials says the same thing, would you be more likely to hear them out?
How often does the image and credibility of the messenger affect your ability to the receive the message?
The source is important if you want to estimate the accuracy of facts being reported ("does that person have the knowledge and expertise to be communicating reasonable accurate information?").
However, considering facts that are well accepted and known, the ability to receive a specific argument will depend as follows:
Everytime somebody is not sufficiently involved in a topic, has not the motivation, time or intellectual capabilities to understand, that person will pay attention to clues technically irrelevant to the actual argument: expertise of person, way of saying things, perceived social status, dress, if what that person says is a threat to you or not, etc.
However, when people are involved in something, are interested, have the time and intellectual capabilities to understand, they will far more likely pay attention to the actual value of truth of the argument regardless of the source.
edit: I would like to add that it does make sense to accept information more willingly from someone who has credentials than from someone who has not. For starters, in order to judge the competence of someone you need yourself to be competent. You cannot possibly know everything and one has to have the humility to admit that one is in no position to judge the expertise of a person without being an expert themselves. Credentials aren't exactly given on a silver plate...