> Ability to link to documents, bookmark, meaningfully use history and save pages to the disk for offline use. Ability of the users to customize standard behaviors without reverse-engineering your JavaScript code. Transparency, which often leads to much, much easier debugging and improved usability. No need to run a quad-core 4GB desktop to use the website.

You can manipulate the page url/browser history from js, and you can use the url to set the application state. History and bookmarks work perfectly well in gmail, for example.

This does take explicit coding. But, if fragments of the page are being replaced as bcx does, then you need to do similar coding anyhow. Otherwise, you're doing whole page loads on every request.

js works pretty well on modest smartphones, at which level network latency is usually the major concern...