About 98% of Trackbacks are spam. Obviously that can’t be addressed with a new spec without breaking backward compatibility. (For added humor, see the front page of the Trackback Development Blog.) The charter of the new trackback abandons the only thing that made Trackback interesting, and incidentally one of the things it was created for, content aggregation and category aggregation and other applications that don’t require a link at all. (Essentially a push application of what everyone uses tags for now, it was way ahead of its time.) In the meantime, they’re re-inventing Pingback with XML-RPC replaced by Atom/REST.