Thanks for the link! I think it's not really possible to derive general conclusions, regardless of the app in question, though. Compared to Gimp or Inkscape, Skype is a different class of application.
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.
Totally agreed that Skype is a different beast than Gimp/Inkscape.
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.
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.