I'm going to bump this as well.
The reason is that I've encountered 2 IRC servers that use message tags to give a message a unique id, and then later on that id is used to reference the line when a state changes.
Examples of this:
The two servers sends an edit event for when a message gets edited. It would be nice to be able to edit the referenced line instead of displaying a new message showing the edits
One of the servers sends a delete event when an op or user deletes a message. Though clients aren't required to remove the message, it would be nice if scripters could remove those lines as they desire.
;; Received message
@msg-uid=123abcXYZ :User!Ident@Address PRIVMSG #example :txet
;; Message was edited
@target-uid=123abcXYZ :User!Ident@Address PRIVMSG_EDIT :text
;; Message was deleted
@target-uid=123abcXYZ :Op!Ident@Address PRIVMSG_DELETE :reason