I guess I should have defined my question better.
1) Fluff: Background Story info or descriptions of the world, its people and how everything interacts.
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3) Rules: How things get done in the frame work of the rule box. By rule box I mean how the complete set of all rules work and what were they designed to work with or work for.
In many cases the background story & descriptions are inseparable from the rules set as the rules are based on a preconception of that. For example RM, and virtually every other fantasy game, is based heavily on Tolkein as is obvious how core races are set up. Core Earthdawn is much different in that orcs and trolls are visually similar but are different in the context of the rules of the game. Cave Troll sky raider??? Blood elves are just wacko and are unique to ED.
The main different here is if the core rules are setting specific, like Earthdawn or Dark Heresy/Warhammer, so the 'fluff' from the above definition is necessary for the understanding of the basis of the rules. RM and many other games while meant to be more generic or not as setting specific can afford to be lighter on the fluff as it isn't necessary to understand the context of the rules. However it is within the context of a preconceived setting that the rules 'make sense'. It just seems that Middle Earth - fancy schmancy tree hugging elves and evil nasty orcs - is the 'default setting' most games start with assumed by most players.
2) Art: To me art is presentation item. To me it is an important item as seeing something interesting that is not words can draw many peoples first and second looks.
Art and presentation make you pick up the book in the first place and holds your attention while skimming it. The Dark Heresy books have a great presentation, artwork (although little of it is actually original) and feel of the game. If I were to pick up a rule book with zero art and simply read like a textbook I would not likely buy it unless it were referred by someone else. Art is an important marketing tool.
That being said too much of it can be a bad thing. If every other page is a full color pic of debatable talent the likewise I won't be likely to look at it further as they are obviously adding fill to expad on the page content (much like the second and third Matrix movies).
Art also serves as an example of what the authors were trying to describe in the rules. Often is seems that when a new system comes out the publishers already have a setting in mind and most likely in production so the art/background fluff in the core books are essentially teasers for the 'new and exciting soon to be published Bob's World by Bob's gaming company'.
So in conclusion, fluff is a necessry component of all core rules set. It really depends on the system, and how setting specific it is, as to where the lines between fluff as 'page filler', 'marketing component', 'example of the rules in action' and 'why the rules are that way' are drawn.
