This line of thinking ignores simple domain specific languages like SQL, which 9 times out of 10 are held in strings in the host language (and thus undergo no static checking because the contents of strings are basically ignored by the compiler. There are other examples too: html files contain nested Javascript, CSS etc. PHP files host HTML. C++ effectively hosts C and assembly.
Haskell hosts a few dozen languages called "extensions" - specified in a {-# LANGUAGE #-} pragma at the top of the file. This one is of particular interest because the language appears to have some kind of extensible syntax which allows these extensions to occur - except when you look under the surface, they're all combined into the same grammar and they all interact with each other, such that other extension developers basically need an entire understanding of all of them to know where conflicts may lie.
Using a framework like LanguageBoxes instead to host these kinds of extensions would allow individual developers to put their own, independant extensions into the language, without having to hack on the compiler and rebuild it.
Also, writing several independant programs and using any IPC to communicate between them is ideal in theory, but in practice is more often unsuitable - because Unix processes are a bloat. They take time to initialize and use lots more memory than necessary for what amounts to running code for a small amount of time and discarding of it. Perhaps if we had a more lightweight model of processes ala Erlang style, this kind of reusability would be practical and not just a good philosophy to follow.