Well you don't, you can just go back to the Start screen and click the app you want to switch to.
Imagine if you had to go to the Start menu (and possibly into the All Programs Menu) to switch to another already open app in current versions of Windows.
But sadly, your suggestion is the currently the only reliable method of switching between Metro apps in the Win8 dev preview because Suspended apps disappear from the Alt+Tab (or is it Win+tab) ring, resulting in infuriating bouts of "where did that g*d*mn app go, I JUST had it open!", and much shaking of fists.
Of course, the alternative; to leave all apps in the Alt+tab ring results in an overflow of apps you used once days (or months) ago, making the feature useless for switching between current tasks. For example, my iPhone task list has 37(?!?) apps in it (viewable 4 at a time). I only ever use that list to kill off misbehaving apps for that reason (or sometimes I get OCD and kill them all).