The value of XHTML 1.0 is that web documents can be processed as XML. XHTML is the easy intersection between the current web and XML.
The value of XHTML 1.1 is the ability to create XHTML subsets, extensions and other variants. This is the platform for extending HTML that people have wanted for years. Murray Altheim, current W3C lead of the HTML Working Group, writes:
... I've got a half dozen or so XHTML variants using the
XHTML 1.1 modular framework, versions that include SVG,
MathML, etc., a version with a DocBook superstructure.
I'm working with the HTML Writer's Guild on an XHTML-
based DTD for the Gutenberg Project ...
The value of XHTML 2.0 will be the change to XLink from HTML's linking syntax. Still modular, with extended forms capabilities and possibly more complex events.