It depends entirely on what I'm buying the supplement for. If I'm buying the Complete Warrior's Guide to Smashing, I'm probably buying it for new feats/prestige classes/whatever, and I want those mechanics to be as divorced from fluff as possible so that I can work them into my game with the bare minimum of effort. I want the feat to be "Death From Above" and not "Shi'Rondel Diving Talon Rend".
If, on the other hand, I'm buying a campaign setting supplement, I want each and every page of the book to drip flavor and fluff. I want to turn every page with a better feel for and understanding of the setting, regardless of whether that page was full of feats or maps or just a big full-page picture of a fantastic location.
These are, of course, ideals. But the difference between The Magic Splatbook and The Eberron Magic Splatbook should be that everything in The Magic Splatbook should be divided into nice, bite-size chunks that I can drag and drop into every setting, while a page in The Eberron Magic Splatbook that doesn't develop the Eberron setting for me is a wasted page.