I proposed an XML-based format to insure the templates passed cleanly between the two applications. Ambitious sites require a god's eye view. They suffer from epidemic link rot, content decay, navigation tangle, and sitcom-caliber coherence. The grassroots school of web design can't help. Site builders are an urgent necessity, not a luxury.
Site managers do the reverse.
They deal in parts and sacrifice the whole to application whim. Yet nothing, in the end, can replace the freedom to take your data and move on to greener pastures if you so choose. PSI extends this freedom to site structure. PSI maps a site not as a collection of parts but as a complete whole. Site structure is preserved in an open format, safe from application-specific chains.