Many big projects, like Python itself, mention a guaranteed ABI compat and yet, sometimes something passes and breaks everything, this happens more than we would like to admit.

Regardless of stable or not, there is the build infrastructure, maintainers time to update CIs to RCs, than stable, etc... It is not a good practice. Folks are using it b/c:

1. PyPI mixes unstable with stable in a very confusing way, sometimes even when folks do not explicitly passes the option to install dev versions;
2. The unstable side is safe from online criticism b/c their breakage is expected. Those building stable packages from the unstable one aren't.