If you can write your application in an efficient way using message passing - that's fantastic! However some applications have shared state at their core and are very inefficient when programmed using message passing.

Are there many such cases where you would use python, though ?

Yes. One example would be scientific computing, which is an area that demands extreme machine performance and which also is frequently done in Python.

Well if it where possible to use python in such a case there would probebly be demand.