That's just for scripting. A proper C API is needed for plugins that do more than the simplest edition operations. At one point I wanted to write a plugin that let you model distance field fractals and would make meshes from them, but there's no way I'll write something that computationally intensive in python. And if I do it in C with a python bridge, it would have meant copying mesh data to and from blender over python. That's not even going into stuff like custom intersectable primitives.

Blender would really benefit from exposing its inner data structures in a way that lets people write high performance plugins.