The nested user elements are a thing we haven't really solved yet. The
problem is that the nested user is not complete, missing some of the
attributes and this must be signaled in some way. A missing id might
not be optimal, but is a strong signal. Also I would like to add that
the ideal way to identify and address the resoures are by their uri:s,
since they are universally unique and you would never have to do any
translating.
We're also reworking the track/set metadata and their relationship to
users, this will probably be implemented within the coming month.