i don't challenge your point on how it should go down.
serving xhtml as text/html and having namespaces with it, makes it "ok" in all browsers, even ie. so, i'm gaining. even if i'm relying on browsers to sort the mess up. invalid html, as you put it, but also invalid functional xhtml.
i'm seeing here apps like lotus webmail. made specifically for ie, with xhtml doctype. that works. that's how it goes down. so what was their motivation, knowing that xhtml will be served as text/html.