Just curious. How often to you go back to old notes?
At some point, everything will get written into a digital scope of work document or deliverable. At that point, most things become superfluous and I don't look back into notes.
Every once in a while I'll want to remember a name or a book and I like being able to have those at the tip of my fingers. Those things are typically in a different notebook.
I take a lot of notes and honestly enjoy it.