Once this move is made/accepted, I would expect the other implementation to rapidly move away from their custom implementations of the stdlib and contribute to the shared code base and documentation. Yes, this places a burden on CPython, but in the long term in benefits *all* of the projects equally by simply having more active contributors.
I would also like to point out that some valuable contributions were made already by other implementations. When talking about stdlib, it's mostly in the area of test suite, but not only in terms of "skip those tests", but also improving test coverage and even fixing bugs. Unicode fixes were prototyped on PyPy first and some PyPy optimizations were ported to CPython (the original method cache patch came from Armin Rigo as far as I remember). So it's not completely "Cpython's burden" only. Cheers, fijal