For me, the issue is not in the details of how the minimized app is implemented (and yeah, I can imagine that WP does some crazy stuff), but more the simple existence of a flexible way to display multiple tasks on a single screen. There are some tasks that simply don't need to take up the whole screen to work; for these, it can be extremely convenient to park them all together in a single display.

In Sailfish, you cannot design an app to work with less than the full size of the screen. Instead, you get the chance to create an additional "cover" for the app, which may take up 1/4 or 1/9 of the screen, and may have one or two switches for I/O. Neither the user nor the designer can control the size or placement of these covers, and the user must keep the full-screen version of the app running at all times in order to maintain the existence of the cover. This just seems incredibly awkward if you actually wish to use the miniaturized interface for any serious work...