We will have a security overview doc up soon, we ran out of time to have it ready today. Long story short, there are pending key rotation states and the latest holder can always re-key the note again. But it is indeed distinct from cash; we can't just state a note has value like governments do, by fiat.
We have to provide users with sufficient information to assess the veracity of that claim relative to the functions of Bitcoin and the information we provide on the note.
> But they have to do that here too, or is the assertion that they don't need to back up the user key necessarily?
Not necessarily; in the condition where a user is confident they are the only ones that can claim, they can place the notes in a safe the same way they might printed Bitcoin keys or Opendimes (I would recommend a fireproof safe).