Right now I'm starting to get into WPF, and after working for quite a while at my internship with web design, I find that the markup structure is great for modern applications web or otherwise. I mean most Presentation and GUI framworks are only minimally reliant on the visual aspects and mainly rely on the manipulation of those presentation objects and time is heavily spent in events manipulating them. I find the markup language style much more efficient than what you'd see if you design a site using other GUI frameworks mainly because you can structure the visual aspects much more cleanly and without (on the surface) relying on generation of containers and their children and doing that whole micromanagement just to create a display. I am sure there are cons but with the way most applications are designed for overall usability and clean layouts I don't see that this could be only recently have been utilized for creating applications.