If the app lifecycle management for Metro style apps isn't working and is causing memory problems, that's a bug/flaw in the app lifecycle management. The solution isn't to rely on the crutch of making people manually close apps, it's to fix the bugs. I don't have to manually close apps to avoid memory issues on iOS or WP7+, which have a lot less than 2GB of RAM.
Apps that "need to be closed because of a problem", for example, should just be shut down automatically. IIRC this is how it's supposed to work already, so if it's not, that's a bug.