I only add stuff to my sketchbooks if I have an idea that needs to be sketched out. Specifically; ideas that are for new characters/projects that I do not already have a file for, otherwise I'd draw on loose leaf paper and file it on completion.
Ah, I see. That would be logical since you're no stranger to drawing digitally. I'll have to give that a shot after I get the hang of not always needing paper to draw.