They still have that terrible multi-window mode set as default, or they finally changed it to usable single-window?
From my personal experience, most people doing graphics usually have a multiple monitor setup. That alone rules out Gimp's single window mode for me.
Putting aside the discussion on Skype, I was answering your "I have no idea why one could find single-window more usable" from my point of view, that is, a newbie user doing the occasional edit/filter/crop.
For such a casual use case, yes I find the single-window mode more usable.
What's "objectively more approachable" for one application, makes zero sense for another.
https://en.wikipedia.org/wiki/Multiple_document_interface
When you alt-tab to the application, all of the child windows come to the foreground. You don't have to alt-tab 4 times to bring all of the various windows forward individually.
I select windows with alt-tab and I can assure you that multi-window mode in gimp is a nightmare.