If Python was so fast as C, the latter would be present in python apps/libraries?
Example: if Python was fast as C would PIL be written completely in Python?
It makes sense to use C modules in Python for:

Performance
Libraries that won't be ported to Python (because of performance reasons, for example) or that use OS-specific functions
Scripting. For example, many games use Python, Lua and other languages as scripting languages. Therefore they expose C/C++ functions to Python.

As to your example: Yes, but Python is inherently slower than C. If both were equally fast, it would make sense to use Python because C code is often more prone to attacks (buffer overflows and stuff).
Is there any purpose for a python application use C other than performance?