I use Evernote personally and professionally with a premium plan. I'd love the ability to select which notebooks/stacks can be synced/viewed per client so that my work computer will never have access to personal notes.
My biggest fear is needing to search for a note in a work notebook for work and have private notes in the search results. I present my screen multiple times per week and opposed to my coworkers seeing how I use Evernote, they don't even know it's installed. It's not ideal for me, and definitely hurts any prospective word-of-mouth advertising that Evernote could benefit from.
This seems like it would be an easy implementation. During install, offer to select which notebooks to sync to this local install, and offer the ability to edit the selection in the Synchronization options. Bandwidth and sync operations would also benefit.
I use tags extensively and keep notebooks very broadly so would perfectly for my use-case. It also aligns with the existing use-cases for notebooks, since sharing is done per-notebook, so I suspect it would benefit many others too.