What is the rationale between creating two books, a d20 Rulebook and a d20 Atlas?

To me it seems like a better idea to create a single tome called the "Birthright Campaign Setting". This is an line with what WotC does with Forgotten Realms, Ravenloft, etc.

Two advantages are:

(1) Everything in one place. Players need both books anyway so why separate them?

(2) We avoid the frustrating "which book to read first" problem for new players. If new players read the Rulebook first, then we need to include various Birthright background info in the Rulebook so that he understands things like bloodlines, awnsheighliens, etc. However, this stuff usually goes into an Atlas. The other ways around, if the Atlas is the first thing to read, then the player won't be able to understand the game mechanics used to describe characters, domains, etc.

I really can't see any serious disadvantages with the single tome approach. One might say that separating the rules and the setting is good design practice, but I don't agree: these things are (at least in the case of Birthright) tightly interwoven. That is, in my opinion, what makes Birthright great. Another might suggest that having two books allows separate releases. Well, there is nothing wrong with releasing the single tome more often as chapters enter beta.

Of course, we can group the Rulebook and the Atlas into a tome later when they are both ready. It does not have to be done now.

I think the Forgotten Realms book has a good design and break-up of chapters (although I don't like setting itself much):

INTRODUCTION
CHAPTER 1: CHARACTERS
CHAPTER 2: MAGIC
CHAPTER 3: LIFE IN FAERUN
CHAPTER 4: GEOGRAPHY
CHAPTER 5: DEITIES
CHAPTER 6: HISTORY
CHAPTER 7: ORGANISATIONS
CHAPTER 8: RUNNING THE REALMS
CHAPTER 9: MONSTERS

Of course, we need to adjust that a bit to a Birthright setting:

INTRODUCTION (includes a brief history and a brief description of unique BR terms)
CHAPTER 1: CHARACTERS (d20 Rulebook chapter 1+2)
CHAPTER 2: RULING A DOMAIN (d20 Rulebook chapter 5)
CHAPTER 3: MAGIC (d20 Rulebook chapter 3+7)
CHAPTER 4: HISTORY (d20 Atlas chapter 1)
CHAPTER 5: LIFE ON CERILIA (d20 Atlas chapter 2)
CHAPTER 6: GEOGRAPHY (d20 Atlas chapter 3)
CHAPTER 7: ORGANIZATIONS (d20 Atlas chapter 4)
CHAPTER 8: GODS AND RELIGION (d20 Rulebook chapter 4)
CHAPTER 9: ARMIES AND WARFARE (d20 Rulebook chapter 6)
CHAPTER 10: RUNNING A BIRTHRIGHT CAMPAIGN (part of d20 Rulebook chapter 8)
CHAPTER 11: CREATURES (d20 Rulebook chapter 9)
CHAPTER 12: ITEMS AND ARTIFACTS (part of d20 Rulebook chapter 8)

The suggested appendices of the d20 Atlas should probably be included somewhere in the chapters above.

What do you think?