I think you have raised a very important point. Most of the edited books are compilation of articles from here & there and have no professional value. Since I was in Library Committee, I can say that these are dumped in the library to utilize the Annual Books purchase budget. They need to be shunned!
First you have to explain what made you think with regards to your assertion as to why all edited books needs to be discouraged. I do agree with you that there are several predatory publishers who charges chapter authors to publish within a book. This leads to poor quality books being floated around.
Edited books can be a valuable resources at many levels. Reading habits, in general, low among many academicians these days. Having a hard copies of these might encourage them to read it. They can be part of someone's personal library on a specific disciplines. It is a valuable resource for teaching undergrad and grad students (e.g power point presentation, assignments, exam prep, etc).
Not all edited books are useless or of a lesser quality. When it comes to become an editor for a leading publishers; until otherwise, someone is an authority in their field, they cannot become one if they have an established credentials. First, publishers would reject their proposal based on their credentials. Their marketing team would feel that the book might not be a good seller. Second, when the invitations are sent to leading people in the field, they will decline to contribute. So, will end up inviting people with lesser repute or someone who hasn't established their academic reputation in a given subject.
Finally, I have edited over 25 volumes for leading publishers. It is a real privilege to an editor for these academic volumes. Books can shape one's life! Certainly edited volumes have their own space in the academic knowledge enhancement. Of course, the readers are the best judge to decide which one qualifies!
For this conversation, I thank the contributors ( Murray Sir, Perumal Sir and Rumbold Sir) as the editors would say often in a preface to their books. Publishing 100 books is a noble cause and demands self-toiling but publishing one book of standrads is much rewarding to our conscience as well as to our pockets, I feel. Here, I am reminded of a quote from the great Einstein in response to a book "Hundred Authors Against Einstein:"
“Why 100? If I were wrong, one would have been enough.