I'm looking forward to having single window mode. Reasons:
1) I occasionally have to use gimp on OSX, which has an absolutely broken focus model. Click-to-focus means that if you want to (for instance) select the crop tool and then use it, you have to click on the tool palette to give it focus, then click on the crop tool, then click on the image to give it focus, then click on it again to start cropping. If you forget the second click on the tool palette you appear to have done something but you keep whatever tool was last selected. It's survivable, obviously, but it's like trying to do surgery with mittens on and a little yappy dog in the room.
2) I also occasionally have to use gimp with a very large number of images, and managing all those individual windows can get troublesome. I once (on a windows machine, admittedly) accidentally hit "enter" in explorer with in excess of 3000 images selected. It took nearly an hour to get back enough control to reboot the machine cleanly. Linux doesn't fare anywhere near that badly, but it can still stumble with large numbers of files.
Multiwindow gimp mostly works great when you have sloppy floating focus and a robust window manager. Unfortunately, on some OSs I'm forced to use from time to time, those aren't a given.