For p-books an attractive cover can make me pick up the book and at least read the blurb, so from that point of view it's good marketing, at least on me. Just take a look at some of Moejoe's covers. These covers makes me want to find out more about the books.
When it comes to e-books, I guess in a minor degree, the same thing applies, even if one is looking at icons on amazon.com or kobobooks.com.
And seeing K C May's book icon for "The Kinshield Legacy" under her signature attracted me enough to chase it up and decide I wanted to buy it. Similarly with P Durrant's "Kai Lung Stories". The covers augment the titles and indicate that this may belong to a genre I enjoy.
But I think the important thing with a good cover design, or icon in the e-book world, is that there is a very narrow line between an e-book and POD, and the book may well come out as a POD book later. And then it needs a good cover. So I'm spending an inordinate amount of time trying to get a good icon for my e-book, to be released just in time to miss the Christmas rush.