I'll explain why an app would do this:
My app (since 10 years..) lets a user compare two workbooks. In order to read all cell properties without errors I need the passwords of protected sheets. I'll also need the passwords if a user decides to "highlight" results. My current approach is trying each supplied password (plus space ,x, xx and xxx ) on all worksheets, which is/was convenient. I'll find a different approach for xl15.
My app always works with form plus 2 windows.. the change from MDI to SDI was a nightmare involving a lot of "hooks/subclassing" to get it solved. This is just a minor hiccup. And in my experience xl2013 isn't all that slow except on the UI side: activating window/sheet/range requires up to 250ms until fully rendered.
I've learned a long time ago to avoid screenupdating (as in "select" and "activate") until the code is ready to return to the user. xl15 does a lot more on a simple selection: the "selection" box is animated and each window has a ribbon with a "few" callbacks. A lot happens on a simple window activate and it ain't all pretty.