Today, I read a bunch of posts about art in gaming products. Stephan Poag often writes thoughtfully on the subject. As one might expect from an artist, he is firmly in the pro-art camp. James Maliszewski tolerates art in moderation. A minority actively dislike art as "a very superficial unthinking gloss for fantasies," but they are asses.
One thing about gaming art that I haven't seen mentioned—perhaps because it's so obvious—is its practical value as an aid to navigating texts. I tend to mentally index gaming books by their images, even when the content of the images doesn't directly illustrate nearby text. It's just a lot faster flipping through a book to recognize a picture than to recognize a block of text.
I suspect that's also why new editions with replaced or rearranged art are so jarring—they disrupt that pattern recognition.
This is not to say that I think of art as a merely a superficial unthinking gloss.