A JSON (or SOAP) backend is only usable by third parties if its API is kept stable. There are far too many devs who redesign their backend request and response formats at the drop of a hat because they think their js client is the only one that matters (a self-fulfilling prophesy) and they can replace it simultaneously. And their responses tend to look like "here's some more markup to stuff into an arbitrary location in the DOM we're using today", not semantically structured (e.g., Rails now has this built into JavaScriptGenerator). A given site can be reverse-engineered, but anything built on that is going to be fragile and short-lived, much more so than when the typical visual rendering desired for a page determined its structure.