Ron's cover sold me "The Painted Bird." I am very interested in how people are treated when they are different and it was clear from Ron's cover that the painted bird was different and was killed because of its difference. I think we all agree it's not the only factor in sales but initially the cover can often catch the eye and sell a book
You are absolutely right. The cover is an important marketing tool but it is not the only one.
I just finished a cover for a traditional publisher (a large house that specializes in science fiction) and a good deal of time was spent in making sure that the cover conveyed exactly the right impression about the book, so that it both attracted and informed the potential reader.