Having a working knowledge of how Win32 works at the lowest level will certainly be invaluable if you are planning on doing Windows development in the future. It gives you a level of insight into things like Windows, Messaging and GDI that are hidden by the time you get to the level of .NET.
I wouldn't recommend you try and use Win32 for writing all your applications, but I feel that any Windows developer would benefit from writing a simple Win32 application using C/C++.
This is less true for things like WPF where there is less dependency on Win32, but just knowing how Win32 works will help you understand or appreciate some of the design decisions in WPF.
