Seriously for a second, take my mobile phone (Samsung Galaxy S2), for instance. If you wanted to display a list of applications in its own dialog or menu over the running app, you could probably do that, but each of the buttons/icons would probably be too small to press. So, this is done in full screen on that device. This is a purely functional decision and is not subjective at all.
A different example. Fast forward to Galaxy Note 10.1 and you'll find multi window mode. Two apps, one next to the other. Enough space to do both, especially when using the pen. Something like this would probably not work on my SGS2.
And then there is the desktop, which uses mouse/glide point as a pointing device (very precise) and has a much, much larger screen. And yet, Gnome insists to treat this as a tablet or a phone by overlaying kitchen sink over my work every time I decide to do even the most trivial "something else". It changes my view (twice) in the process, forcing VNC to repaint large areas. It moves my windows around (expose), although I never wanted to do this. These are nonsensical usability regressions for a _desktop_ UI.
PS. If you wanted to see how a pretty good UI for one device is terrible on another, download Profimail for Android. I used Profimail on Nokia's Symbian for a long time and it was pretty much the best IMAP capable mail client on that platform. On an Android phone with no keyboard, it is pretty clumsy. The UI is all wrong.