There are more reasons to implement in C than just speed ;-). In my code I have two usecases for using PyDict_GetItemString, both not in performance critical code
1) Look up a hardcoded name in a dictionary. These could be switched to the Id API if that were public (and I might implement something like the Id API myself).
2) Look up a name that is passed in as a "char*" from C code that's outside of my control. This is a real use case for a PyDict_GetItemString API and not easilty converted to another API.
In PyObjC the majority of calls to PyDict_GetItemString are in the first category, while most of the latter would probably be better of using a PyDict_GetItemBytes API. Although I definitely to not propose to add such an API to CPython.