This method uses XmlHttpRequests to get and receive packets from a BOSH implementation on the same domain as the connecting site. The only way to run in BOSH mode is to serve your site from the same domain as the BOSH implementation (same-origin policy); this can be achieved by using an HTTP proxy or a Servlet filter.
Script Syntax allows connections to a remote BOSH server without requiring a BOSH implementation on the same domain as your site--it is immune from the same-origin policy. This is because it uses script tags rather than XmlHttpRequest.
This method also allows XMPP-enabled applications to be run locally though a plain HTML file or Adobe Air.
There are no performance reasons or otherwise to avoid using Script Syntax.