I always wonder what is someone's vested interest in the message, what do they have to gain or lose, what agenda are they seeking to further. If I agree with their agenda, I don't care about their credibility, because I can use my own to advance the cause. To me it becomes a matter of how many credible people you can win, which makes the message more palatable to others. Most people want their spokesperson attractive and with a Colgate smile, makes them more likely to give you the time of day.