I guess you have to look at what WPF offers over Webforms. The only thing that comes to mind is more possibilities GUI wise, but that again make WPF a bit more complex. Most Webforms applications aren't made to look good, they are meant to perform certain operations in an easy and understanding way and there WPF doesn't really give you much over Webforms.
Maybe silverlight could give wpf a push. There are a lot of interesting possibilities with full screen siverlight apps(Photo apps, office apps, report apps...) and they would of course be easily ported to WPF apps.