Twitter is a perfect example of a "thick client". It's slow, bloated and buggy. When one typo is made in the JavaScript "bundle", the whole site implodes, leaving the user with simply the navigation bar. When JavaScript is turned off, most functionality is lost. I couldn't even log out after disabling JavaScript because "/logout" pointed to a hash.

JavaScript being enabled is not an axiom. The client is a very unstable platform when juxtaposed with the server. A lightweight client facilitates flexibility.