Taglines and quotes from reviewers are a useful sales tool in a bookstore. A casual browser can be persuaded to buy a book on the recommendation of an author or celebrity that they already admire. On an ebook cover the tagline is rarely visible (especially during the process of browsing and purchasing) because it will be viewed in thumbnail. That kind of review and recommendation text is much better displayed on the webpage next to the cover, not on the cover itself. However, there are reasons to keep the tagline on the cover: so that the ebook cover will match the hardcopy cover; so that the work of cover design doesn’t need to be done twice; in case the browser decides to click-through and view the large image.
I’m undecided whether the illegible tagline on the cover has a negative effect on aesthetics of a book cover in thumbnail. Consider the cover of Firestarter (which was designed before ebooks but is still a useful example). The title, author name and image are all clear in thumbnail, but the text below the title is just slightly too small to read, which is both tantalising and irritating to the viewers squinting down at it. Likewise, in The Celestine Prophecy the text is illegible and would probably be better replaced by an image of a pyramid. In Cold Mountain the rather obvious information that the book is a novel is slightly too small to read comfortably, forcing the viewer to work harder to understand the cover.
However, there are covers which I think benefit from the tagline, despite it being illegible. In Gone Girl the original cover is rather empty without the tagline and the sticker. There is nothing to draw the eye around the cover or to keep it in the image. The sticker adds some much needed shape and contrast to the composition, while the tagline helps to fill the blackness.
In Stardust the quote from William Gibson is too small to read. However, if it were removed the composition of the cover would be ruined by the wide-open negative space that is currently filled by the quote. It’s better to keep the quote, just to balance the image, than to reject it for being illegible.