As Darren New once posted, "Multiple Document Interface, or some such. Each document gets a sub-window inside the main application window. A left-over from when windows took a large percentage of resources to make work, back in the Win3 days, so it was more efficient to have one document in the same window." Also, Windows users want to iconify an application with all its windows, and their window manager doesn't provide for what X11 adepts regard as a "wm group".
Robert Heller says "this is the window-manager-in-a-window hack".
Sam Tregar's saMDI [L1 ] looks well-behaved. It depends on stooop.