[blacksmith_tb]

Sure, it's not hard to hit cmd-q instead of cmd-w, I think the general question is "what's the benefit to me, the user, of having an app still running (and in memory, until it gets paged out to disk) without any windows open?" Obviously there are some kinds of apps that it makes sense to have running in the background all the time, but we're talking about text or image editors, say. I find it to be sort of annoying.

[Tagbert]

For document apps, the idea is that you are working on a document. You can then close that document and open another document without the app closing when you close the first document. It’s not really about having apps running in the background. It’s just a slightly different way to look at documents and apps.

[blacksmith_tb]

I guess that suggests an over-optimization in favor of working on more of the same documents eventually - but I might not do that any time soon... I opened an app, edited a document in it. When I am done, unless I have other files open in that app, it seems like it might as well be completely closed, until I launch it again or open one of the document types associated with it? I guess some users may only work on a few kinds of files, and in a few apps, so they might benefit from slightly quicker load times; otherwise having a bunch of things running on the off chance I use one again seems like a waste of RAM.

[Tagbert]

…and in that case, you would close the app.

Conflating closing a document window with closing the app seems like a strange constraint.