All Evernote users are free to use tags and/or notebooks and/or stacks, in whatever combination that they wish. But you cannot use arbitrarily nested notebooks, because they don't exist in Evernote. For that, Evernote would need to implement them, across a number of different applications and their servers, and API (which is used by 3rd-party developers as well). It's more support, more testing, more mode documentation, more complexity all around. It may not be rocket science, but none of this comes for free, either. Seems as though they don't think that it's worth it. See Minus 100 Points for