There was a bunch of discussion about all this a while back, in which I think these points were addressed:
However, in some cases the C locale is a normal environment for system services, cron scripts, distro package builds and whatnot.
Indeed it is. But:
if you run a Python (or any) program that is expecting an ASCII-only locale, then it will work jsut fine with any ascii-compatible locale. -- so no problem there.
On the other hand, if you run a program that is expectign a unicode-aware locale, then it might barf unexpectently if run on a ASCII-only locale. A lot of people do in fiact have these issues (which are due to mis-configuration of the host system, which is indeed not properly Python's problem).
So if we do all this, then:
A) mis-configured systems will magically work (sometimes)
This is a Good Thing.
B) If someone runs a python program that is expecting Unicode support on an properly configured ASCII-only system, then it will mostly "just work" -- after all a lot of C APIs are simply char*, who cares what the encoding is? It would not, however, fail if when a non-ascii value is used somewhere it shouldn't.
So the question nis -- is anyone counting on errors in this case? i.e., is a sysadmin thinking:
"I want an ASCII-only system, so I'll set the locale, and now I can expect any program running on this system that is not ascii compatible to fail."
I honestly don't know if this is common -- but I would argue that trying to run a unicode-aware program on an ASCII-only system could be considered a mis-configuration as well.
Also -- many programs will just be writing bytes to the system without checking encoding anyway. So this would simply let Python3 programs behave like most others...