[aplaice]

XMonad's[0] default behaviour of being able to separately tell a program to go "fullscreen" (i.e. hide most UI) and control what portion of the screen its window takes up is extremely valuable, since many programs don't give you the option of separately hiding their UI, and it's only possible by making them go fullscreen. However, being able to hide the UI is actually most valuable when there is more than one window on the screen... This is especially important when you have a small (or average-sized) screen, but it's also useful on larger ones to avoid distractions.

Getting the "normal" fullscreen is just two keyboard shortcuts away ("fullscreen" the program and assign its window the entire screen), rather than one. From an intuitiveness point of view, being able to hide a program's UI and change its window size separately is arguably more intuitive — according to the separation of concerns, the former is managed by the program itself and the latter by the WM.

PiP is really nice, but videos aren't the content for which getting a distraction-free experience is most important...

[0] I'm sure that it's also possible with other tiling WMs such as ratpoison, stumpwm, awesome etc.