In reply to Weston Ruter.

Agreed; especially since 4.1 the customizer is really a JS-driven framework in terms of UI and can accordingly provide the fast single-page-app experience that users are coming to expect. The customizer is also natively built around the concept of comprehensive extensibility, which Calypso seems to lack currently.

The bigger benefit of the customize API is that live preview is built in and it's (increasingly) a frontend-based interface. Calypso is missing contextual live previews and isn't particularly exciting to me as an end user as a result.