If two paths actually work, then invariably some web sites will choose to link to your page with http://xyz.com/foo/bar/baz.html and other sites will choose to link with http://www.xyz.com/foo/bar/baz.html. Each version of the URL has its own "referrer count" in a search engine, making the page seem less popular overall.
While search engines could do extra work to identify URLs that are aliases of one another, they're not obligated to do that. And even if a search engine does what you want, other tools may still be confused by duplicate paths (e.g. an archiving system or other link crawler).