In terms of coherence, the W3C has made it clear that all future markup language developments will be based on XML. It stands to reason that HTML, W3C's flagship markup language, must also evolve towards XML.
One of the reason for adopting an XML-based language, is the breadth of XML tools which are available. These tools can really make a difference.
Last year, I participated in a project to extract search information from portals like Alta-Vista. Our software would request an HTML page from Alta-Vista and parse it to extract the search information. Most of the development time was spent parsing the HMTL page. Had Alta-Vista use XHTML, we could have used one of the freely available XML parser and saved a lot of development effort.