Not super long ago I tried to launch the idea of making a #commonplace to create a sort of community commonplace book on Hubski. The feedback I got back largely said that it wasn't a great medium, and as time has gone on, I've become convinced of that too. Personally, I don't see the need for a single physical notebook for a commonplace anymore. I have a notebook I call Log in Evermore largely for quotes, and an older Notepad doc which utilizes the .LOG feature to create a quick journal and commonplace. The use of browser bookmarks, and an app called Pocket provide space for larger sets of information to be stored. I'm sure there is something to be said for writing it down, but I don't see that quality as justifying my time, space or effort in this regard.
Very much understand your perspective on digital alternatives, but in the same way I keep a written journal, I can see why I would feel more 'involved' with a written commonplace book. Perhaps it's because my major use will be to capture things I wish to learn and reaffirm, or perhaps it's that I'm more likely to browse through previous entries as I'm capturing new ones. Up to now, I've used Evernote for this, but I'm definitely guilty of dumping content into Evernote that I have never read again, or very rarely review. I think if the 'barrier to entry' is higher (the effort of writing vs simply hitting a button), then I will be more selective in the content I capture, and more likely to review. Also, I just have a thing about notebooks. :)