I recently read of a review of an RPG that criticized it for having black and white art as well as color art. I think this is misguided, for several reasons.
First, I like black and white art. It takes skill to do properly, and can be very effective at setting a mood or providing inspiration. The RPG that the review mentioned above was about used color art for full pages or double page spreads, and I thought it was very effective and attractive. I also found the use of black and white art on the other pages, mostly taking up part of a page, very effective and attractive. I don't mind the mix, if it is done in a manner that harmonizes well.
Second, I don't mind RPG materials that contain only black and white art. Again, if the art is good, then it improves the work, and is welcome to the eyes. Some RPG publishers simply don't have the money for full color art throughout the work. I don't think it is fair to criticize their work simply on the basis that it isn't full color. In fact, if the work is well written and has no art I'm more likely to enjoy it and get what I want from it than if it is badly written and has good art.
Fourthly, if the work has pages that are completely covered from edge to edge, especially on the paper used for the best reproduction of color art, the ink is much more likely to smear and leave ugly tracks from normal handling.
Fifthly, fully covered pages often have background art that reduces the readability of the page. For instance, the brown background to the first page of chapters in D&D 3E and 3.5E, and the broken brown lines under the text of the other pages in the chapters detracts from the readability noticeably.
Sixthly, when working with maps for RPGs, while I find judicious use of color to make things stand out is useful, I find that maps that are fully colored, while impressive to look at, are much harder to adapt and to record changes on. When I run “B2 Keep on the Borderlands” or Jennell Jaquay's “Dark Tower” I can write notes directly on the map and draw in changes such as collapsed sections or new excavations. I can't do that with full color printed art — the changes don't show up well or in some cases at all, depending on the paper and the ink used. This is less important if working with digital images, because you can always use sharply contrasting colors for text and line drawings, but full color “painted” battle maps are harder to adapt for most people because the skill it takes to modify it and have it looks good is much greater than most people have.
All in all, I don't think color art, especially full color pages, is a requirement for good RPG books and materials. It is impressive when it is used well, but can actually be a drawback in some cases.