I really like good art in game books (heck, there's a print of Todd Lockwood's "elven chain" from the 3.5 DMG on my wall right here)...but, would I pay *more* for a book if it had particularly good art? That's a tougher question, and I'm not sure the answer would be "yes" for me, especially when we're talking about a PDF.
That said, one of the biggest things art can do, when done right, is create a feel and style for the book / setting. Larry Elmore, Jeff Easley, and Clyde Caldwell certainly helped shape what Dragonlance is for folks; Tony DiTerlizzi surely did the same for Planescape, and (for me, at least), Todd Lockwood, Sam Wood, and Wayne Reynolds have helped to define a feel for 3E.