When they invented C to replace assembly language, people where probably asking: "is it worthwhile to learn assembly language?" The value in knowing both was being able to drop to assembly to do the things which were impossible to accomplish in C (eg. trigger an interrupt).
The same can be said for Win32. There are some things which are impossible to do in C#. If you didn't know the win32 api, then you would dismiss some things as being impossible. However, once you know what you are missing, in those rare situations, you would be able to "drop to win32" and do them.
Another way of looking at it is this: programming is all about being able to think in multiple levels of abstraction at the same time. For example, if you know your language uses immutable strings, you don't write an algorithm that adds a single character to one 10000 times, because it will be slow. If you know the win32 api, you will be able to think about how each line you write in C# is actually implemented and that will help you write better code.
