"How long before an app will actually delete your file when you say "Open..."? Sure that could happen, but why would anyone do that? And especially in a open source world where anyone could just fix/improve it?"
That's a ridiculous example - nobody would ever do that because it is functionally illogical. However, the details of window management are very subjective, to do with taste, opinion and preference. Different people have different ideas (and strong opinions) over how things "should" be.
We are talking about the same software community that ended up with a whole GNOME fork based around the order of the OK/Cancel buttons, remember?
"Window managers" answer the question correctly here by saying "the correct behaviour is what the specific _user_ wants".