I like the post it has a lot in common with the approach taken on for command line UNIX applications. Each command line app has a well defined responsibility, the OS provides a simple set of capabilities that allow these applications to communicate with one another. Using the applications and the 'glue' provided by the OS users can perform far more complex tasks. Adding in the shell means that applications can be developed just using the existing applications and OS communication.
In the world of web applications Sitemesh provides the glue roughly equivalent to the OS glue provided by the command shell. By developing applications in the progressive enhancement style allows web applications to be glued together to provide a cohesive web experience to the user.
* Each web application is partitioned into its own space. (loose coupling)
* 3rd party applications can be incorporated into the user experience more easily