If it’s not open source then it’s a non-starter, it doesn’t matter how cool it is. This is not an ideological free software stance, it’s a recognition that science has embraced the utility of open source tools and rejected black boxes. It’s essential for reproducibility and transparency in research. It’s no longer enough to provide your code in a paper supplement or on GitHub; the entire chain must be available for inspection, including the language itself.

EDIT: One common exception to this principle is the use of proprietary C or Fortran compilers for high performance.