I do not see why. The multitasking view is just the equivalent of the task switcher on a desktop, with the additional ability to use a couple of shortcuts.
They would still be useful, even in case you could have multiple visible windows. And the fact that you can have mutiple overlapping windows does not mean you only have to use them; Most of the time, on a desktop computer, I am looking at a maximized window anyway.

The problem of resizable windows, is that it would be hard to interact with them on a touch screen without wasting a lot of space, but a tiling window manager would be nice.