I think a media type parameter is a great idea for application/xml. Unfortunately,
the +xml convention has won the day, and it is embedded in internet RFCs, including the
registration procedure one, so it has little chance of flying now.
The *main* objective of the media type is to identify the semantics of a format, so that is covered by the +xml usage.
But there is no software that I'm aware of that supports identifying / treating anything registered as +xml as xml.
For example, browsers don't know what to do with anything transmitted as application/atomsvc+xml for example.
Parameters would be a help though, especially if one could agree on a standard one like 'profile'.
This has been proposed for atom:
http://tools.ietf.org/html/draft-wilde-atom-profile-01
I don't know if that proposal will move forward. I like it though.
Cheers,
Peter