Comment 5 for bug 435792

An application that does not need users to know whether it is running does not need a "Quit" item because it wouldn't do anything useful. In applications that have "Quit" commands, all tasks stop and (usually) some valuable state is lost. Both of those are engineering deficiencies. But in USC, installations and removals carry on regardless of whether the window is open, and there is no other valuable state to retain (or, for that matter, to reset).

Furthermore, implementing a "Quit" item would actually be harmful, in that it would cancel all current tasks when usually that wouldn't be what the user had intended. (The next logical step would be a confirmation alert asking if you're sure you want to cancel all current tasks; but wanting to add a confirmation alert is a big clue that the original design is wrong.) If you do want to stop current tasks safely because you need to shut down quickly, that can and should be handled by aptdaemon itself interacting with Gnome Session (that way it would work for Update Manager too). And if you want to stop all current USC tasks for any other reason, that could be handled more obviously by a "Cancel All" button in the "In Progress" screen. <https://wiki.ubuntu.com/SoftwareCenter#cancel-bar>

Ubuntu has never used Alt F4 to quit applications, but rather to close windows. Even in Microsoft Windows, Alt F4 is normally for closing a window; it quits an application only if the application has a Multiple Document Interface (child windows inside a parent window), which is relatively uncommon in Windows and doesn't exist at all in Ubuntu.

I agree it would be nice to standardize in this area. Ubuntu Software Center is consistent with Evince, Epiphany, and Eye of Gnome, but many other applications still have vestigial "Quit" items. Unfortunately for the kind of simplification drive necessary to get rid of them is neither easy for volunteers to agree on, nor sexy for companies to work on.