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.