Reader development uses a reader-centred definition of quality. This is the mission statement which explains what reader development aims to do:
The best book in the world is quite simply the one you like best and that is something you can discover for yourself, but we are here to help you find it.
We all know from our own reading that it is possible to have a deep and satisfying reading experience with a book that is generally considered quite light, which may well not be in print in ten years' time. It is also possible to have a poor reading experience with a book that has been accepted as brilliant. This doesn't necessarily mean that the critical evaluation of these books is misguided or that there is something wrong with you as a reader. All it means is that you and the book weren't right for each other at that time, something prevented the book from speaking to you.
Our reactions to a particular book are shaped as much by who we are as by what the book is. Every reader will have a different experience of the same book depending on their personal history, prejudices, and what they had for breakfast!
The quality of the writing is not the sole determining factor of the quality of the reading experience. Writers would love to have that degree of control! In practice, the reader will determine when to stop and start the book; how much effort to make; whether to concentrate fully or to be distracted. And even the reader is often not aware of what is conditioning their response.
Reader development starts with a respect for the individual reading experience. Each reader is the judge of their own best book. Reader development will always seek to encourage people to try something different or new to them - it's about stretching boundaries and opening up possibilities - but the final judgment on whether it was worth it is down to the individual reader.