> Is it a real use case? Why would anyone use a RHEL binary on Debian? Debian already provides the full standard library.

I'm not talking about the standard library obviously.  I don't remember my original use case exactly, but I must have been compiling a C extension on a system and expected it to work on another.

> C extensions of the standard library are tidily coupled to CPython. For example, it may be dangerous to use a C extension of Python 2.7.5 on Python 2.7.15.

I don't believe that.  Binary wheels uploaded to PyPI seem to work fine regardless of the exact bugfix version.

> Third party C extensions distributed as portable wheel packages using the stable ABI is different use case.

Most wheel packages don't use the stable ABI.  They are tied to a Python version such as 2.7, but they don't differentiate between e.g. 2.7.5 and 2.7.15.  We don't break the ABI between bugfix releases.