First, I apologize for my inconsiderate posting behavior. I yell at my staff all the time for doing stuff like that!
Second, I'd like to respond to your last comment, "The act of explicitly closing it is much more user-friendly."
More user-friendly in what sense? As other users have mentioned, after switching to a new window, you have two (or more) windows open. As you've pointed out, more windows = more memory consumption. And as myself and others have pointed out, more windows = more clutter. Like many here, I prefer the single-window UI approach.
Your response is, "You can simply close/minimize the window so you dont see it?"
"simply" relative to what? It's certainly not as simple as switching to a new window, while closing the previous window in a single click. As TheUser1 explains, it's currently a three-step operation. Not only is it more steps, but it causes me lose my focus each time I switch back and forth between windows. I find it highly distracting and it breaks my train of thought. If I'm switching to a new window, I want to immediately focus on my intended task in that window, not have to manually switch back to the previous window, then close it.
Perhaps the strongest argument is that when this feature is enabled, it would force users to work in a more resource-efficient manner. You constantly talk about how lean and efficient Orion is, but the current behavior of leaving windows open by default is contrary to this philosophy. Whenever possible, Orion should assist users in minimizing its memory usage. Automatically closing the previous window would accomplish exactly that.