I'm not sure what the obsession is with W3C validated XHTML, here. They're ebooks. They're not going to be served up to a browser by a webserver. Nothing wrong with making sure it passes ePubCheck's or FlightCrew's requirements, but W3C really has nothing to do with it. I certainly strive for W3C validated xhtml within my web apps, but ebooks are a different beast.
But I understand not wanting to jump through hoops to create a mobi that probably won't meet a client's expectations anyway.