Small applications tend to get bigger over time. If you follow the principles in this book then this growth process will be much easier and quicker to implement.
Probably the biggest thing against this book is the problem of convincing people that they need the information that it contains. The vast majority of people programming applications for the web wouldn't give scalability of their applications the slightest thought. There are lots of moderately large applications out there where the developers would have gained significant benefit had they known about the information in this book before their application started to grow and there are certain to be many more in the future.
What this book will achieve for those who read it is that their future application development will be both easier to maintain and will easily adapt to further growth in the future.
This is not a book for beginners as the book itself makes clear right at the start. Once you are past that beginner stage though and starting to develop significant applications then various sections of this book will gradually start to make sense and become more and more applicable to your development work.
The only real flaw in the book is in the order in which the information is presented as with the way that the topics are structured some of the information presented in each section will not be relevant until your application grows beyond a certain size and for many their applications will never get that big. The content of the book itself is not as scalable to meet the needs of the different readers as it perhaps might have been although at least with the order in which the information is presented you are made aware of what future concerns that you may have with each topic and how you would then need to handle them.
An excellent book for any intermediate to advanced web developer regardless of the languages that you are developing in.