Metric implies there is some way to objectively measure it. I'm not sure if that's true.

But almost everyone thinks the same as you do, and yet ads are huge business. How are you affected? I don't know, but my first guess is brand perception, regardless of your self-diagnosis. If a company is advertising a certain time-limited sale, some of their value will come from conversions taking that offer. But some, maybe most, is the brand impression that people get over time. Think "I never heard of that" vs "Oh yeah, I've seen that somewhere".