GParted seems to be the more robust and feature-rich application of the two, is there a reason as to why GParted is not installed by default in Ubuntu?
A while back I've read somewhere on the net that Disk Utility (Palimpsest) is a udisks graphical front-end included in the gnome-disk-utility package, that gives the opportunity to set up an encrypted partition easily, with a couple of clicks, whereas with GParted you need to use the cryptsetup command line tool.
Can't tell you what the developers had in mind, but the way I see it, an OS shouldn't give you the richest tool available for a certain goal, but the tool most people need in an OS.
By "most people" I mean most people that are part of the destination audience the OS is aimed for, which in Ubuntu's case are probably daily Internet + Office + some Multimedia users, or in other words, the average home user.
Packing a tool that few people will use regularly is a bad design, as it grabs disk space unnecessarily for people who don't use it. Most home users don't need to and don't want to use the intensive features of GParted.
I believe Gimp was removed for the same reason. Most home users do not manipulate images, and when they do from time to time they don't need all the features in Gimp. So why waste their disk space?
Besides, most of GParted's advantage comes into effect only in a live session anyway, because you can not manipulate mounted partitions. Live CD is the natural place for GParted. A few times I even loaded Live CD before windows installation to get GParted for partitioning :)