The main advantage of HTML5 are the new amount of build in features which reflect most of the todays common requirements for web based applications.
Why XHTML 1.0/1.1 failed so far? It mainly was much to strict for the web community - the web and also the world is a non perfect place and therefore HTML5 is much more suitable to fit into this world than the XML based approach of XHTML can provide.
Does this mean XML for the web has loose and does not make sense at all. No there is still space for the XML based standards beside HTML5:
XML serialization of HTML 5 with stricter parsing rules
- Polyglot XHTML (see http://dev.w3.org/html5/html-author/#polyglot-documents)
- Easier Reuse content for different channels using XSLT / XQuery
- Retrieve content as XHTML and extract only dedicated parts (views) required for different use-case
- Store and request the content using XQuery based infrastructure
A good summary of Polyglot XHTML and related XML based alternatives for HTML5 can be found here: http://www.xmlplease.com/xhtml/xhtml5polyglot/