I've found the knowledge gained from various "manifestos" and other writings of criminals to be immensely helpful in my own work in comparative criminology - but more than that, in understanding how to approach people who have similar ideas and writings (as a teacher for more than 40 years, I've run into quite a few disturbed persons who are clearly writing along the same lines and I think I can actually make some positive inroads and interventions).
I know lots of psychiatrists and psychologies who feel the same. I've also posted in a vague manner about what I believe are cases of we teachers, counselors, etc., actually successfully keeping worse things from happening. You don't have to believe that some of us are actively involved in this field, but psychiatric nurses, all by themselves, are amazing in the types of intervention and care they can provide through various means. There aren't enough psychiatrists to answer phones/provide support all day long, but mental health workers do study and talk about the writings, youtubes and utterances of their clients as a way of understanding what to do about these disturbed people.
It's going to keep happening, but in my view you can't convince me that it wouldn't happen more if those of us who work in these fields are kept in the dark. My own mentors were people like Donald Lunde (interviewer of Kemper and others), Peter Rosenbaum (psychiatrist working with criminal clients), and Irving Yalom (focused on personal narratives as a way of aiding psychiatric patients, among his other many, many contributions). They've all used these materials to write books that are required reading in psych programs around the nation (and the world).
In my view, all of this has helped. Indeed, Lunde's early work on serial killers is often overlooked but in fact, resulting in major re-ordering of how LE viewed investigations into serial killing. The writings of the crazy serial killers helped Lunde construct an incredibly detailed timeline of the planning of the crimes of a serial killer in Santa Cruz - which actually led to helping convict him.