Mini-languages basically bring greater structure or different forms of expression. They definately have a place, but I think they can be over-used.

HTML templating, though, is one place where I think they have value. If you believe that there shouldn't be any significant logic in the template files, then an enforced mini-language is good. Also if your template writers are html types using dreamweaver, then a well designed mini-language designed to look like tags (cf. Apache::ASP, Aolserver).

Another benefit can be a different form of expression. Not that SQL is a mini-language, but as an embeded language it holds benefit because some things are better represented by declarations.

One pet peeve about the over-use of mini-languages is config files. There are SO many config file formats, and you often can't do what you want, so you write bogus scripts to read and edit them for you (cf. cobalt qube). I once blogged that Tcl is an ideal config file language: it is simple to embed; simple to learn; and if you don't need any evaluation in your config, then the file will look like a simple text format config file.

(Of humerous note: the thing that prompted my blog entry was discovering, to my painful amusement, that the sendmail.cf format is turing complete...)

I guess my point here is not that mini-languages are bad, but that there are too many poorly concieved re-inventions - if you think you need a mini-language, why not use an existing one like ASP or Tcl.