A number of community members use word processors for I-D authoring because they are familiar with them and because they have good support for collaborative editing.
The Template for Internet-Drafts and RFCs for WinXP MS Word 2000+ comprises an MS Word template and a Perl script for converting your resulting .docx file into RFCXML. This is fully documented in RFC 5385.
RFC Tool converts .docx files from MS Word into RFCXML. This is documented in draft-hallambaker-rfctool.
xml2docx generates an Office Open XML (.DOCX) format document from an RFCXML document.