Each point of view has its own merits. Certainly the paper point of view has intuitive simplicity and is acceptable for applications where it meets the needs.
The protocol point of view can come close to encompassing the paper point of view as a limiting case. In particular, it does so in the following circumstances:
As the complexity of messages declines to a single payload (perhaps with a few attachments)
As the mutability of the payload declines to some standard format that needs little or no canonicalization
As the number of parties and amount of processing during message transfers declines
As the portion of the message intended for more or less direct human consumption increases
In these cases, the protocol ...