They don't appear in the task switchers because Windows would need to re-launch the app in order to regenerate an image for them, which would break the entire purpose of fast task switching. And "keeping a thumbnail" around doesn't work either because now Windows is maintaining a bunch of bitmaps for no reason whatsoever other than to try and stop the Taskbar looking broken. And it has to do so even if I never use the desktop at all, just in case.
Have you never seen taskbar thumbnails that are comprised of just the app icon? No need to start the app and generate the thumbnail. That said caching the thumbnail is a great idea despite your hyperbole to the contrary. Windows caches a ton of thumnails, icons, and the like. Adding a few more wouldn't kill it.
And that becomes pathologically stupid when you consider how it affects someone who only uses Metro apps because now task switching will continually attempt to include apps you potentially ran months ago and are unlikely to ever use again. Apps that the system has already decided aren't useful to you right now and closed. This puts the burden of task management back on the user again, defeating the point and effectively forcing the Win32 lifecycle onto apps. In order to usefully use the system the user has to actually spend time closing closed applications, a design that would literally be beyond ridiculous.
You certainly would be an expert when it comes to "pathologically stupid". You are so utterly fixated on what the OS does under the hood to the app you miss the metaphore. If I had files, pens, papers, and my phone on my physical desk and all of a sudden things started disappearing that would be frustrating.
Months ago? Hmmm... I don't recall leaving apps open that I don't need for months at a time. More hyperbole to support your rabid claims.
I'm sure the OS would have no problem closing a closed app. It's only you my friend that seems to have this problem.
You can't see the problem because you can't seem to consider any usage beyond your own, which is almost entirely focused around using traditional desktop applications almost exclusively. Not everyone is using Windows in the way you are.
So how many people are using Windows as a traditional desktop today vs. Windows as a touch device?