Plenty of things do. Such as web analytics tools & plugins that can only work in a non-xhtml-compliant way.
My favourite was a google tool (can't remember what it was - google website optimizer?) that required you to use some godawful <script> construction that was necessarily broken. And you'd have thought google would know better.
That space before the closing slash is actually not allowed in XML, but was required for browsers that couldn't interpret XHTML. XHTML was broken from the get-go; the only virtue it had was that it taught a generation of web developers to be consistent in their markup.
(By the way, since sibling nodes have no specified order in XML, there's no reason why one paragraph should have followed another on a web page consistently, and the <ol> was an oxymoron.)
My favourite was a google tool (can't remember what it was - google website optimizer?) that required you to use some godawful <script> construction that was necessarily broken. And you'd have thought google would know better.