The reason why it makes sense to use an entirely different language for the templates is quite simply to avoid using Perl. You may be very proficient in writing Perl code, but quite often the people who deal with the templates (and in general the 'View' part of the equation) are web designers. Those people are more likely to be better at HTML and CSS, and it therefore makes more sense to teach them how to use a simple template syntax (% s.name %) instead of teaching Perl (print $s{name}).

Even if all your web designers are Perl hackers, I think it still makes sense to learn a different templating language just for the sake of code clarity. I find it much easier to do all the hard work in my Perl in my Controller class, shove those variables in the stash (I use Catalyst), and then simply print and loop through the variables with the Template Toolkit.

All in all, TT is a very powerful system, and a language of its own. I once designed an entire web page that didn't need any "real" dynamic content, so I didn't have to write one line of Perl code: the whole thing was developed using Template toolkit and the ttree helper application.