Chances are that neither the client nor the agency will ever know very much about what role the ad has played in sales or profits of the client, either short-term or long-term.
If there are signs that Americans bow to the gods of advertising, there are equally indications that people find the gods ridiculous. It is part of the popular culture that advertisements are silly.
The effectiveness of advertising depends on the amount and kind of product information available to consumers... advertising will be more successful the more impoverished the consumer's information environment.