I'm going to be obnoxious and repost my comment from a couple pages back as I think it's still pretty pertinent to the discussion:
I'd say as soon as you have a persistent taskbar (at least with the current Windows taskbar design, where it becomes unusable with too many apps open), and a prominent close button you've established the expectation of manual task management. That's a fair enough tradeoff for the power the taskbar gives you (though it might be interesting to try and design a new taskbar that didn't require this tradeoff - it would probably require separate sections for pinned and unpinned apps, since to scale to app usage > size of taskbar the unpinned section would have to be presented as a LIFO stack rather than a FIFO queue), but I really hope this won't eventually be the only mode available for "PC" form factors.
In general, I agree with Andy that you lose something valuable by doing this and that's a shame, but I'm not sure it's not a necessary tradeoff. But I definitely don't agree this makes store apps pointless - you still have the modern APIs, you still have the isolation, you still have the deterministic install/uninstall, you still have contract support, you still have ARM support, you even still have suspension and constrained backgrounding on minimize. And of course for device classes or users that don't have this setting checked you still have the full benefits (and drawbacks) of the app switching model that existed before.