Partly - the Unix philosophy. Since you can swap out your window manager on X11, you can have your windows managed in any of a wide number of ways. If an application tries to manage subwindows on its own, there is a very good chance that it will do so differently than your global window manager. Assuming that there is a reason you picked the global WM that you did, it's rather disconcerting to be forced to use an inferior WM to manage Gimp's windows.

Add to that the multi-desktop, multi-monitor aspects (although detachable docking panes can help a lot with that).

Desktop environments on Unix systems on which GIMP was primarily supposed to run have always had virtual desktops, and with virtual desktops you can easily distribute the little tool windows to one desktop and the image windows to other desktops (or physical monitors if you fancy), which would not be possible in the single-window mode.

GIMP's multi-window interface is excellent when you have multiple monitors. Couple that with a good window manager and it's a lot better (IMO) than a single window with tools and other dialogs inside.

A multimonitor setup is great but it hardly answers the OP's question. When this decision was made multimonitor setups were rare...

I have my window manager manage the windows. I can use all my usual keys and shortcuts to modify/move/alter the windows. Eg mousescrolling over a window's title bar to reduce it to just its titlebar if I do not need it (there is a shortcut too, I forgot it though). Being able to make windows always in front. Being able to have multiple images open and move/resize them with alt+(left|right) mousedrag.

I like having only my image on screen. With multi-window mode, you can press the tab key to make tools windows appear/disappear.

When you're used to change tools via keyboard shortcut (and thus, don't need tools window most of the time), it's a great benefit.

It's useful to be able to put your tools on one monitor, and maximize the image on the other.

Would be cool to have a single window of image, and a separate single window of tools.

You can already do that : you can choose to only display toolbox window, and then add tabs (like a layer tab) in the toolbox window.

Ah, best of both worlds. I'll have to try that.

you CAN do that, dock all the dialogs in one single window

- you can move away the docks and have more screen estate for image editing; - press TAB and the docks go away, you have even more screen estate; - is easier when you work with more than one image at a time, for example using the clone tool you take samples from one image and move them to another or just when use one image as reference while drawing another.