There’s a term for such reasoning: begging the question. The question begged is whether the underlying assumption — bringing a message makes you de facto credible — holds. Hint: It doesn’t. Else, our lexicon would not include replies like “nonsense,” “my foot,” the ever-handy sardonic “yeah, right,” and a host of less polite terms meaning roughly the same thing.
If you want people to believe your claim, back it up. When rational-thought champion James Randi claims that homeopathic “remedies” are nothing of the sort, he explains medical science and shares controlled studies. He even swallows a bottle of so-called homeopathic sleeping pills before a live audience, following which he stays not just awake, but alive. Never does he assume people will believe him simply because he is the one bringing them the message.
Just as advertisers should be able to prove claims when selling to consumers, ad agencies should be prepared to prove their own claims when selling to clients. When your ad agency tells you that a concept is likely to work, ask why they think so. If they have evidence to back them up, great. But beware hollow, glib answers that beg the question.