Lots of people don't use any "desktop environment." I just use ratpoison, for example.

Well, call it a "window manager" then, it still should handle this. But that's just my opinion of the responsibilities of WMs/DEs/whatever you call them, and I understand that some don't.

No, handling screen layout isn't the window manager's responsibility. That's what external tools like xrandr and its various GUIs are for.

I use dwm on my laptop, and I have 2 external monitors.

Every morning I had to make an xrandr call when I plugged them, and another one at the end of the day when I unplugged them to make my window manager aware of my screen layout.

I was tired so I automated the thing.

I agree that good software should do this but unfortunately multi-monitor configuration software isn't always very good. My Ubuntu 15.10 system with Unity desktop decides to move the taskbar/dock thing to the other monitor every time I turn the other monitor on in the morning. There is nothing in the Screen Display GUI that seems to allow me to stop this from happening. Out of frustration I've resorted to running the xrandr command a few times (once is sometimes not enough!) to fix it each time. This tool would at least automate the task for me and seems useful.

My laptop running i3 is often moved between a dock on my desk and used as a regular laptop, I have to say I've been looking to write something like this but it fell by the wayside. Very useful IMO

This issue is the very reason I use KDE Plasma, I never came across another DE that handled multi-monitors correctly.

Gnome doesn't do this. Without fail, every time, it puts the dock on the wrong external display. Every. Time.

feel your frustration , I just never take mine of the dock cause i can't be arsed anymore.