Steve B said:
A good example:
BI's WFRP. The main book is just over 250 pages and presents a complete game. They have a forthcoming Companion (128 pages) that adds random stuff. Lots of random stuff that's totally unnecessary, but will be great to have. Each section on it's own is too tiny to stand alone, but it all fits well in a Companion. And it's coming out about 2 years after the main rules.
Not everyone prefers things that way. People that do not require the rules in the RQ companion include those who just need to pick it up for character generation for third-party games produced under the SRD, and they don't want to pay more for a book twice as large full of things they don't really need. You should have added in your example that the WFRP rulebook costs nearly twice as much as the RQ core rulebook, which is a lot of money when all you wanted it for is the rules for generating Characteristics.
What Mongoose have done is produced something halfway between a very basic rules pamphlet and a comprehensive rulebook, with enough information to play a fairly complete game on its own without getting bogged down with information not everyone will need in their game.
(actually, if it were down to me I'd have dropped the section on Rune Magic as well but you can't please everyone). There's a little bit of everything in order to explain how the basics of the system work, to give some example monsters to mess around with. It's also a useful annotated and extended edition of the RQSRD which works well to clarify how to do what you want to do with it.
It's also a great marketing decision (a statement which I'm sure will draw some unwelcome comments), as it's a relatively cheap book on the shelves which people can pick up, hooking them and drawing them into the game. Far too many lines don't sell because the core book itself is too much of an investment to risk the money on.
Just to illustrate how good a decision that was by Mongoose, I've already sold nearly half as many copies of RQ as I have of WFRP since it came out, and it's only been out for four days.
I don't really want to come off as knocking WFRP here, as I like that too. The thing is, I pick up WFRP to play in the Warhammer world, and am happy that it is "complete". I pick up RQ as a small, more generic, and simpler to play rulebook, and hopefully one for which there will soon be a fairly widespread range of settings for.