0) Smartphones and PDAs from days of yore requiring styluses (or a very skilled digit) and low resolution screens.
1) The iPhone which had a higher resolution screen with increased hit target sizes to support finger-based multi-touch input.
2) Androids, BBs and WinPhones released afterwards with generally the same limited screen real estate and same multi-touch finger input.
3) More recent high-res smartphones, iPads and other devices that could reasonably support several "screens" simultaneously, but due to supporting existing App Stores, kept the same UI guidelines.
Multiple "windows" on the same device only really make sense if you're on an iPad or larger touchscreen and even then only really if you're using a stylus since one hand will be holding the device 75% of the time you're using it. Also fitt's law would suddenly make those nice edge-positioned buttons that an App placed into very frustratingly small hit targets. Other considerations - how would you support Android's "back" button without cause a hell of a lot of confusion?
The point at which multiple screens makes more sense is if you have a keyboard case on a largish tablet. At this point, it's effectively a netbook and could be running a real desktop OS (Macbook Air 11" / Chromebook).
On Windows tablets it is perfectly ok although I highly dislike metro's lack of windowing management:
Customize your screen with 3 metro apps (e.g. twitter, browser, Xbox music), switch to another full screen metro app (flipbook), Switch back to twitter and there you go, metro forgot you customized the screen and shows twitter full screen. What king of crap is that 18 months after windows 8 launch.
Apart from that though I am extremely impressed with the Surface 2 and multitasking with outlook, word, browser & apps is a given.
Re r00fus to me the back button, also on Windows phones, is a major ui design failure. On touch buttons it could change depending on context but it's impossible for hardware buttons.