The risk with a completely open registry is that any
fool might want to register headers related to his
favourite religious fanatism, or spammers might want
to register headers favoring their web sites.
That's one risk. The risk I'm more concerned about is the guy who
does something that substantially changes the way that mail works,
and this creates a rift in the installed base and degrades
interoperability for everyone.
( And no, I don't subscribe to the theory that the market will
select the optimal result in the long run. )