As a framework developer it is essential to have an easy way to individually test the appearance of and the interaction with each custom control. I really hate it when I first have to open five different screens before I finally get to my new control so that I can test it. Not only is it annoying but it also costs too much time. When doing UI work you constantly make changes to your controls and look at the impact of those changes. If a single change roundtrip (code, launch, test) takes 5 minutes then you can only squeeze 12 change iterations into an hour, making slow progress. If it takes 1 minute then you are looking at 60 iterations and fast progress. As simple as that.