Let me just add a story to show how an alternate python implementation may be useful for users. As I was porting datetime.py to 3.x, I saw many failures from pickle tests. It was not easy to figure out what was going on because C pickle code was calling buggy Python and pdb was unable to trace the full chain of calls. To work around that, I added sys.modules['_pickle'] = None to my test run and there you go - the problem was found in minutes. I am sure that someone debugging his tzinfo implementation, for example, may find datetime.py easier to work with.
The story may be a bit self-serving, but I was against this "feature" myself, but now I see enough use that I am actually working on it.
Yes, the work is in the sandbox, but I want to have py3k working version before I announce it.