While you certainly can close Metro apps manually, there's no direct need in terms of resources to do so. Metro apps will be suspended when not on the foreground and removed from memory when free memory gets tight. When switching back, the app is resurrected from the grave.
Such Jesus apps do depend on the developer not being lazy / retard. While MS checks apps during the winstore certification process, sometimes apps slip through that do not react to such events correctly. Using those apps is pretty annoying.
I often need to force restart metro apps gone wild, and I do so in the task manager (didn't know about the finger trick). These are first party apps like Mail also (to be fair, I have to do this in iOS also, no app is perfect!).
I don't use metro much on my desktop, just laptop and surface RT unit. The network sucks here in china, and most metro apps don't deal with it very well.
Sure, it's the developer being a retard. Not WinRT's fault for a model which pushes more complexity onto the apps than more straightforward multitasking would, in addition to the behaviors being difficult to test and discover.
Such Jesus apps do depend on the developer not being lazy / retard. While MS checks apps during the winstore certification process, sometimes apps slip through that do not react to such events correctly. Using those apps is pretty annoying.