In terms of cover art selling books, I can tell you this is ABSOLUTELY true. Go ask any publisher. It's a little known fact that any paperback with a swastika on the cover will outsell the same type of genre without one (and no, I'm not trying to be offensive). There's a strange fascination with the symbol in the American marketplace (go look in the war fiction section of your local bookstore - swastikas everywhere!).
Pictures do a lot to sell product; maybe not to you, but on the aggregate it does. Does crappy cover art reduce sales? I dunno, but good cover art definitely helps it. Think about it: an average joe goes into a book store and has two books he knows nothing about to choose from. He can pick the one with a cool picture on the front or the other that has no cover or a really bad one. Which do you think he'll pick (based on these facts alone)?
I definitely agree that there is something to be said for having "neutral" covers for the core rulebooks, if for no other reason that to avoid alienating potential customers from the books that form the building blocks of the entire game. But other than the core books, I'd put more emphasis on good cover art, especially when the market is now flooded with 3rd party products. Good art is a clever way to differentiate yourself in the market.