I use office because that's their biggest money making client application suite. You would think it gets the most UI usability focus of any of their products, so it must mean something when they don't use any of the UI frameworks they push for everbody else.

Visual Studio is an interesting one, huge effort to get working on WPF, which needed improvements to WPF for them to use it. Surprise dog fooding your UI framework causes improvements pretty quick.

Imagine what WPF would be like if the Office team used it, might be cross platform.