Probably a very niche use-case: I have an ultra-wide display that I
split into two using `xrandr --setmonitor`, and I want the workspaces to
stay in place when the split ratio is adjusted.
Furthermore, this fixes workspace reshuffling when a virtual monitor is
added for screensharing a portion of the screen
(https://news.ycombinator.com/item?id=41837204).
Can't think of a scenario involving just physical screens where this
would be useful. Those are mostly added/removed, so if anything, one
might wish to preserve the workspace that is currently being showed, but
that would require knowing the output name (only available via RandR,
not via Xinerama). If someone physically moves their displays around and
then invokes `xrandr` to update the layout, this might very well do the
right thing, but I don't think anyone moves their displays around often
enough to be annoyed by xmonad reshuffling the workspaces. :-)
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