The best reason to use any template language, in any system, is in the readability and functionality a template language brings.

Readability: only if you have a high ratio of static/dynamic code; functionality: only if restricting is a benefit.

The problem is, it doesn't scale. For any HTML entity, any visual reference, it can affect another entity or reference. If I want to apply css, js and what not, I don't have a single document that contains everything in a single view. What's worse is, to find any particular piece, I have to go backwards and figure out what calls what.

What do you mean by 'scaling'? A template doesn't save you from "entities affecting each other". You have a single document only in the case of trivial pages with sparse dynamic content. If you have e.g. inclusions (of other templates), reading gets equally difficult, an you have to go back to look where each bit comes from.

And if you have a different language, e.g. that of Template::Toolkit, you have to go back to the calling code to see what every token means.

    [% INCLUDE debugtxt msg="file: $error.info" %]
[download]

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}