Old guy that I am, I'm used to RPG books that were instruction manuals full of text blocks and nothing else. In the begining it wasn't especially worrying to me to not have a lot of art, but art adds so much that now having seen how effective it is, I think of it as a necessity.
Good art provides it's usual service: inspiration. A jumpstart to the imagination is a very valuable thing. Done well, it evokes a feeling of time and place that really gets the old creative juices flowing.
Art and illustration is vital to an RPG, especially in a setting book. I think the various armor and weapon illustrations are needed in the basic books, if only to ally the state of education most people receive in them. Most people know what a sword is, but you should see the blank looks some people get when you talk about banded mail or a sai. True story swear to God, I had one guy think a mace was a chemical weapon.
A setting book needs to take the generic focus of the game and tighten it considerably. I want to see the various fashions for the Sword Coast. I want to see priests robes. I want to see what a Baldur's Gate coin looks like as oppossed to a Zhent coin.
More than the simple presence or absence, the
quality of the art is a big factor, as is the choice in setting, mood, tone, etc.
The differences between this
and this
are palpable. Same monster, same stats, but presented in such a way as to fire the imagination. Can you imagine being that guy on the front lines facing such a thing? Imagine the terror he has to feel? Yeah. Big difference.