I think the consequences of unredirecting are too much of an engineering headache to make it worth the performance gain. When you unredirect a window, you either have to disable compositing entirely, or else figure out a way to work around the limitations of unredirecting. For instance, what do you do if a modal dialog window pops up in front of the fullscreen app? Plenty of "fullscreen" applications would want this; for instance, for generating a dropdown menu in many widget toolkits, they create a new XWindow.
I like that. Problem is and always has been the drivers.