When adding screens to a notebook, it's highly likely that we'll be working on the same notebook until we finish adding screens. It would be fewer clicks and more intuitive if one could simply click the button without having to expand the drop down list.
So I imagine once you select a notebook to add the first screen to, every other screen would default to that previously selected notebook until you change it again. Sort of like how the saved list shows up.
This would (hopefully) help avoid confusion where folks in the org who are new create multiple notebooks (one screen per notebook) not realizing they can add to the same one to get a presentation style roadmap.