This book is for people who really want to understand the 'why' and 'how' of Ajax.. as all Enterprise developers should. Frameworks are great timesavers, but you should be prepared to write your own extensions and fix bugs as needed. To do this effectively, you will want to have a deep knowledge of how to work within a framework, not just how to use the framework's API.
The framework we develop in the book is very lightweight and mainly for demonstrative purposes - so you can learn how these frameworks tend to be built.
The book isn't all about a framework though. It discusses a lot of peripheral topics like security, usability, and design.. which applies whether you use a framework or not.