To add another perspective, the same question has been asked for other languages. And in many cases, the "original language" was chosen as the default template system.
For example, PHP itself is already a template system, but other template systems like Smarty have been created. I always felt that's a little over-done, but it does allow better enforcement of the separation of presentation (most of these templates allow embedding of the original language anyway, but discouraged). Some recent frameworks in PHP have chosen to stay with PHP for its view component, at least by default, notebly CakePHP and CMS systems like Drupal and Joomla.
Ruby made the same decision in Rails, where the default template is rhtml/erb, where ruby code is direclty put in html.
Java has jsp where java is directly embedded, but then people have been arguing that servlet plus a template system like Velocity is philosophically better.
Back to perl, TT does give me a much warmer-fuzzier feeling. I also like the fact that I can, and do, use it outside of the web environment.