1. This will increase the size of type object. Most types are not numbers, not collections and don't support the buffer protocol, thus save memory for tp_as_number, tp_as_sequence, tp_as_mapping and tp_as_buffer. This also can increase the time of type creation.

2. This will make harder testing that the particular protocol is supported (e.g. see PyIndex_Check or PyObject_CheckBuffer). And this will break any third-party code that does this directly, without using C API.

3. Calling function instead of executing inlined code after checking can increase stack consumption and slow down execution (see the usage of tp_descr_get).