Conclusion
The benefits of interoperability are broader than what I've discussed here. You could completely overhaul an application's user interface with WPF, but hook it up to back-end logic that was already in place with the old user interface--even if that logic was unmanaged code. This could be done with a number of techniques, such as using C++/CLI, PInvoke, or COM Interoperability.
Still, there are clear benefits for having an all-WPF user interface rather than a hybrid one. For example, in a pure WPF user interface, all the elements can be scaled, styled, and restyled in a similar fashion. They can be seamlessly overlaid on top of each other. In addition, you don't have to worry about mixing resolution-independent elements with resolution-dependent elements. A pure WPF user interface also opens the door to being able to run in a partial-trust environment (depending on how you separate your back-end logic).
