LOL no, I agree with you, Mare. I think a banner is more part of the overall presentation of the story, nothing more. It serves a similar purpose to a summary - something to attract you to the story. I equate it to the cover of a book. When I'm browsing in a bookstore, an interesting cover is what makes me pick up a book... but then I automatically flip it over and read the back, cause the summary is what will really determine whether or not I think it sounds interesting. The picture is just something to catch my eye in the first place. I see the point of having them, but I definitely don't think the story as a whole should be judged on the basis of a banner. Like I said, if we were doing something with overall presentation, that's where I could see a banner factoring in.
Personally, I make them because I enjoy making graphics, and I'm a visual person, so it inspires me to have a visual to go with my stories sometimes.