As a GM, I care about having good art in my RPG books for three reasons.
1. Good art tells me about the setting, and about the tone of the game. White Wolf books are a classic example of this kind of art: moody, atmospheric, almost universally excellent. One flip-through, and I know roughly what kind of game the designers had in mind.
2. Quality art gets my players into the setting. Similarly, evocative art that’s matched well to the game gives my players lots of cues about what to expect, and helps get them in the mood to play.
3. I can show things to my players. This is crucial for monster books — I can describe a beastie, then show it to my players to cement that image, ensuring that we’re all on the same page. Ditto with locations, NPC portraits and the like.
(There’s a fourth reason, too, of course: good art is more fun to look at. That’s not GM-specific in any way, though, so I left it off the list.)
For these three reasons, bad artwork can keep me from buying a gaming book. With monster books (which I love), poor or limited artwork will keep me from buying them, period. In setting books or other sourcebooks, bad or limited (low quantity) art won’t necessarily keep me from snagging a book, but it’s definitely a factor.
Where do you stand on this one? And are there other reasons why art in gaming books matters to GMs?