And also what Dave said: an author writing a pre-XEP and then walking away
before it becomes a "proper XEP" would be a problem if the pre-XEP was just a
wiki page or something else without assigning copyright to the XSF: even if
someone wanted to work on the pre-XEP and advance it to a "proper XEP", that
wouldn't be possible unless the original author assigned copyright to the XSF,
but that may never happen.
I realised there's a much simpler case where keeping pre-XEPs outside the IPR policy causes horror. If multiple people collaborate on a pre-XEP, then they all have a claim on the copyright. It'd be slightly harder to move the pre-XEP under the IPR policy than to relicense a typical open-source project - you need to contact every author, etc.
Dave.