I think there are quite a few reasons. Mine are: 1) Ability to render in alternate ways to improve readability and accessibility
Please be more specific on "accessibility". Which groups of people are currently excluded, or would be much more included with XML documents?
2) Ability to cross reference documents
That benefit only appears if all, or a significant proportion, of the Internet Drafts are in XML or a similar format. That's not what you proposed.
3) More uniformity in, e.g. references
In what way is that helpful? Are there kinds of references commonly used now that don't work well?
--Paul Hoffman, Director --Internet Mail Consortium