We don't *have* to use a converter function, no.  But consider: someone somewhere will have to convert a PyObject * into a Py_ssize_t, also accepting None.  Doing it in a converter function means we can easily share the code with bisect_right, which should presumably behave the same way.  It also keeps all the argument parsing logic in the Argument Clinic domain instead of having some in AC and some in the implementation.

Is there some drawback to using a converter function?