Why is Gimp no longer included in the installation image by default, despite the fact that Canonical has moved away from trying to make the image fit on a 700mb volume?
GIMP was removed for a variety of reasons, including that it took a lot of space (as it ceased being included by default in 10.04, back when an Ubuntu ISO was still under 700 MB).
In addition, it was also found that most users did not need most of its features and that its interface was too complex for simple tasks. It was thought that most people wanted to edit photos in simple ways, so it was replaced with F-Spot (which itself was later replaced by Shotwell). For those who wished for a more serious image editor, GIMP was (and still is) easily downloaded from the Software Center. It is likely for these last reasons that GIMP has not been re-included as part of Ubuntu's default software suite.
For more, see this OMG Ubuntu article discussing the changes.
I really don't think they have thought much about it, and have just tried to keep it as small as possible to allow for much faster downloads and such, though I do have to admit; it would be nice to have a checkbox upon installation that says "download full application suite" when connected to the internet that would download stuff like vlc, gimp, and maybe some relatively full featured trackpad management system. :P