Mason isn't designed to be a templating system. It's a Content Management System, similar to Asgard and many others. It's a mark of its strong design that it can be used solely as a templating system. And, like the others, it does CMS very well.

I don't understand this statement at all. Mason is very much not a CMS! A CMS is an application. For example, Bricolage is a CMS.

Mason is a templating system plus an application framework, mixed together, which is where it differs from HTML::Template or TT. For example, with Mason, you can plug it directly into mod_perl without writing any code but Mason components. You don't need to write code to marshall your data and pass it into Mason.

In other words, Mason is entirely self-contained. Of course, it doesn't have to be used that way. David Wheeler's MasonX::Interp::WithCallbacks module lets you do some processing before Mason steps in. I'm also working on something similar(-ish) that works with Mason. There's also Mason support for Maypole now, too. So you're certainly not obligated to use Mason and only Mason for an app.