I have a proprietary language at work that I would love to give the 'JetBrains' treatment. (Code completion, structure awareness, jumping to definition, etc.) The MPS platform supports all those wonders plus a great hybrid editor that allows faster, less error-prone method of content entry.
I could even use the template system to make the AST visualization look exactly like the text-based DSL we work with AND have MPS output text files that look just the same.
But the base language is what's implemented in the wild and what is read in to the engine and not everyone would be willing to change over to an AST-only system. I would need a way to 'open' a text file via some import method that turns text into the AST with what MPS tries so hard to avoid - parsing - and then 'save' by exporting that AST back out that original text file from whence it came.