It seems nice enough, but I think it's not needed. For a template as simple as your example, most if not all the existing is fine. For more complex examples (only include this section if...) then either one will have to switch from your to a more complex one, with a need to rewrite code and template.

And this approach looses something I really like about Template: solid visual clues to the web-designers about what they must not change. E.g.:

    Username: <input type='text' name='username' value='' />
[download]

vs:

    Username: <input type='text' name='username' value='[% default.use
+rname %]' />
[download]

And then all the other things you will have to implement to get a fairly complete template system. I toiled with making my own, but found I still had loads to learn about the existing templating systems.
But that's just my $0.02
Edit: typo in placing the default username in the name attribute.
Update 12 hours later: I might be in for some serious reconsiderations.