Problem is, absent good design review and an agreed-on specification,
reuse of existing fields can cause interoperability problems.
Precedence, Priority, and Return-Receipt-To are all examples.
Sure, you don't register a header unless there is paper somewhere that
describes the proper way to use it. That could always include an internet
draft in the case of a provisional registration.
We need more than documentation to use headers widely, we need good design.
And experience suggests that fields designed by individual implementors
aren't suitable for widespread use.
Keith