Full-screen application switching has a far far greater mental-cost than having two windows open at the same time.
As an XMonad user with a 15.4" screen, I disagree. For example, when programming, I find it quite easy/fast to alt+1 to my browser for documentation, and alt+8 to go to my 8th workspace, where I usually have a couple terminals open with vim. Or sometimes I'm switching between those two and Gimp on workspace 6.
This doesn't really require XMonad, just having workspaces and quick shortcuts for moving through them is good enough. Most applications are easier to use when you give them the whole screen (at least smaller screens). But yeah, if you're using some sort of taskbar/dock to switch between applications with your mouse, it's probably easier to just have both open together.
This is true, a very-fast screen switcher mitigates the cost heavily, maybe even negates it entirely. Alt-tab never did it for me, too unpredictable. On OSX, while developing with a single monitor, I pretty often split my things between 4 desktops, and it's great.
On Windows, on the other hand, nothing switches quickly. Ever. And it takes a long time for applications to "wake up" after even short periods of non-use. Nothing but a second monitor has ever saved me.
On the iPad, currently, the situation is far worse than even Windows users suffer through. Double-tap the home button, and click the application (maybe scrolling between), which isn't in a static location. That's a non-screen action, followed by a change in the UI, followed by a touch, followed by a comparatively-slow transition. I liked someone-here's suggestion of gestures solving this, but single-tasking on anything without a separate keyboard is a total efficiency-crap-shoot everywhere, from what I've seen.
The issue that I have with desktop switching on my shiny new MacBook is that OS X switches focus between applications, not windows.
Here I've got vim and Firefox open, with the focus on vim. I switch to another desktop to glance at some documentation (since the Firefox window on the first desktop is for the application I'm developing), switch back and begin typing. But alas! My key strokes are logged to Firefox, because that was the last application I had focused, rather than to the last window I was using on the desktop.
Aside from that, once I added some basic tiling functionality[0], it's not too bad working on it, even for a someone as used to a tiling wm as me.
The examples you mention (e.g. programming, then looking something up in documentation) only require you to switch between apps rarely. In those cases, it makes sense to let each app take up the whole screen. The additional screen space each app gets is worth more to you than the mental cost of having to switch between screens.
In other cases (e.g. writing a cover letter or a job application while looking up information about the company on their website), you constantly refer to two different apps, so it makes sense to allow both to exist on the same screen.
Agreed. I suspect this notion that you incur mental cost from screen-switching comes from those who have not used a tiling window manager. Once you get used to this, a desktop is clumsy/painful to use.
I have the same layout every day: screen 1 is tmux with several xterms, one running vim; screen 2 is the browser; 3 is email; 4 is remote connections; etc.
I constantly toggle alt-1 and alt-2 to jump between vim and the browser and it takes no mental effort. Why not put them side by side? I could, but I prefer full screen.
[Your down-voter could perhaps re-read the guidelines.]
As an XMonad user with a 15.4" screen, I disagree. For example, when programming, I find it quite easy/fast to alt+1 to my browser for documentation, and alt+8 to go to my 8th workspace, where I usually have a couple terminals open with vim. Or sometimes I'm switching between those two and Gimp on workspace 6.
This doesn't really require XMonad, just having workspaces and quick shortcuts for moving through them is good enough. Most applications are easier to use when you give them the whole screen (at least smaller screens). But yeah, if you're using some sort of taskbar/dock to switch between applications with your mouse, it's probably easier to just have both open together.