Very often, mini-languages are meant to constrain the user for various reasons. Some reasons I can think of are:
- Performance - HTML::Template is a lot faster than Text::Template. The price is the lack of features. But, that was the conscious design decision when samtregar wrote it as a reaction to the Template Toolkit version 1.
- Maintainability - I have worked with Text::Template, and it's ugly. You have Perl code in your templates - this means that you can do anything. This includes the stuff you shouldn't be doing in templates per Separation of Concerns, like calling out to the mainframe to count something.
- Ease of use - not everyone knows Perl. In fact, most people who know Perl don't know Perl very well. As someone who has studied programming, in general, and Perl, in specific, for a long time, I have a very clear idea of how I want the Perl code to look and what I want it to do. If I have a web designer working on my templates (which I love to happen because it is part of MVC), I don't want to have to teach them Perl. In fact, I don't want them to even know that Perl will be calling their templates. I want them to work with HTML and CSS, plus a very simple mini-language like HTML::Template's or a subset of Template Toolkit's. That's all they need to know, which makes their and my jobs easier. They have to worry about less to get their job done and I have to worry about less to maintain what I've done.
So, I hope you see that Separation of Concerns can actually be used to argue for the use of mini-languages. It's not as black-and-white as you seem to hope for.
As an aside - I'd like to note that making an argument based on the thoughts of one person, however respected, is a dangerous proposition. I have no idea who Rob Nagler is, and he sounds pretty smart based on the quotes you've shown, but he's still one person with opinions, some of which are both right and some of which are wrong. A better argument would be based on quotes from a multitude of sources, at least some of which have been seen to argue with some of the others. That would provide your argument with a sounder footing and it would be more compelling.
Being right, does not endow the right to be rude; politeness costs nothing.
Being unknowing, is not the same as being stupid.
Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.