IMO - Covers are meant to grab your attention as you are browsing through rows/aisles of books. A good cover will also "set the tone" - give an impression of the book and what it's about. I don't know how many books I've completely ignored because of a bad cover...unless, of course, it is
sooo bad that I have to laugh at it...
It would seem like a lot of extra work to make an image of text...