From Gnome Shell Tour page "One way the user can switch applications is the improved Alt+Tab dialog that displays all open applications" So does the dash.
From Gnome Shell Design page "The application switcher provides a mechanism for quickly switching windows that is familiar to users of Windows and OS X" For me personally its sounds like "Hey, look we can do it too". If Gnome Shell tends to be the truly "next generation desktop" it has to dissociate from such old ideas as an appswitcher in its current form. Why old? from wikipedia "Alt+Tab is the common name for a keyboard shortcut that has been in Microsoft Windows since Windows 3.0 released May 22, 1990; 23 years ago"
At the moment we have a dash, an overview mode and the appswitcher on top of that, and all of them in a way (switching from one task to another) copy each others functionalities.
My view on that instead of a separate element as the appswitcher, integrate it in the dash since its already displaying running applications.
From Gnome Shell Design Page "The dash indicates currently running applications and acts as an optional quick launch and favorites facility".
And when it comes to the window thumbnails of the switcher they should be integrated in the overview.
The switcher in the dash is app centric (ALT+TAB) while the overview is used to cycle thru opened windows (ALT+~) . The benefit of this approach is:
1. Take advantage of users visual memory (user already knows where his windows are located in overview) 2. Make Gnome Shell simpler by deprecating an extra visual element 3. Create a workflow where the dash and overview complement each other
There is a bug regards this matter